Vehicle driving assist system

ABSTRACT

A vehicle driving assist system is provided that calculates a risk potential indicative of a degree of convergence between the host vehicle and the preceding obstacle. A first driving assistance control system controls at least one of an actuation reaction force exerted by a driver-operated driving operation device and a braking/driving force exerted against the host vehicle based on the risk potential calculated. A second driving assistance control system controls the braking/driving force of the host vehicle such that a headway distance is maintained between the host vehicle and the obstacle. A transition detecting section detects a transition of operating states of the first and second driving assistance control systems. The control adjusting section adjusts the control executed by the first and second driving assistance control systems when a transition of operating state is detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/751,805 filed on May 22, 2007. The entiredisclosure of U.S. patent application Ser. No. 11/751,805 is herebyincorporated herein by reference.

This application claims priority to Japanese Patent Application No.2006-142713 filed 23 May 2006. The entire disclosure of Japanese PatentApplication No. 2006-142713 is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a vehicle driving assistsystem configured to assist a driver with respect to the operation of avehicle.

2. Background Information

Various vehicle driving assist systems has been proposed to assist adriver with respect to the operation of a vehicle. An example of avehicle assist system is disclosed in Japanese Laid-Open PatentApplication No. 2000-54860. In this publication, the vehicle drivingassist system increases an accelerator pedal reaction force such thatthe driver can rest his or her foot on the accelerator pedal duringautomatic cruise control. With such a system, the driver can overridethe automatic cruise control and accelerate the vehicle by depressingthe accelerator pedal during automatic cruise control. Another exampleof a vehicle assist system is disclosed in Japanese Laid-Open PatentApplication No. 2004-17847. In this publication, the vehicle drivingassist system executes headway or following distance control and changesthe accelerator pedal reaction force in accordance with changes in thetraveling situation of the vehicle.

SUMMARY OF THE INVENTION

With the conventional driving assist systems just described, it isdifficult for the driver to execute driving operations in accordancewith his or her intent when the system is switching between an automaticcruise control or a following distance control and accelerator reactionforce control.

In accordance with one aspect of the present invention, a vehicledriving assist system is provided that basically comprises a precedingobject detecting section, a risk potential calculating section, a firstdriving assistance control system, a second driving assistance controlsystem, a transition detecting section and a control adjusting section.The preceding object detecting section is configured to detect apreceding object existing in front of a host vehicle. The risk potentialcalculating section is configured to calculate a risk potentialindicative of a degree of convergence between the host vehicle and thepreceding obstacle based on a detection result of the preceding objectdetecting section. The first driving assistance control system isconfigured to control at least one of an actuation reaction forceexerted by a driver-operated driving operation device and abraking/driving force exerted against the host vehicle based on the riskpotential calculated by the risk potential calculating section. Thesecond driving assistance control system is configured to control thebraking/driving force of the host vehicle such that a headway distanceis maintained between the host vehicle and the obstacle. The transitiondetecting section is configured to detect a transition of operatingstates of the first and second driving assistance control systems. Thecontrol adjusting section is configured to adjust the control executedby the first and second driving assistance control systems when atransition of operating state is detected by the transition detectingsection.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure:

FIG. 1 is an exemplarily system diagram of a vehicle driving assistsystem in accordance with the present invention;

FIG. 2 is a schematic perspective view of a vehicle in which the vehicledriving assist system shown in FIG. 1 is installed in accordance withthe present invention;

FIG. 3 is a schematic side elevational view of the vicinity of anaccelerator pedal of the vehicle illustrated in FIG. 2;

FIG. 4 is a graph plotting the driver's requested driving force versusthe accelerator pedal depression amount;

FIG. 5 is an elevational view of a display device of the vehicle with ameter;

FIG. 6 is a diagram illustrating the control state transitions of afollowing distance control and an RP conveyance control;

FIG. 7 is a flowchart showing the processing steps of a vehicle drivingassistance control program in accordance with the first embodiment;

FIG. 8 is a flowchart showing the continuation of the processing stepsof the vehicle driving assistance control program shown in FIG. 7;

FIG. 9 is a pair of diagrams (a) and (b) showing a method of calculatingthe risk potential;

FIG. 10 is a flowchart for showing the processing steps executed inorder to calculate the risk potential;

FIG. 11 is a graph plotting the reaction force command value versus therisk potential;

FIG. 12 is a graph plotting the repulsive torque exerted during the RPconveyance control versus the risk potential;

FIG. 13 is a pair of diagrams (a) and (b) showing examples of what isdisplayed during the RP conveyance control and the following distancecontrol;

FIG. 14 is a flowchart showing the processing steps of a vehicle drivingassistance control program in accordance with a second embodiment of thepresent invention;

FIG. 15 is a flowchart showing the continuation of the processing stepsof the vehicle driving assistance control program shown in FIG. 14;

FIG. 16 is a graph plotting a coefficient for modifying the requesteddriving force versus the accelerator pedal depression rate;

FIG. 17 is a graph plotting a modification control continuation timecoefficient versus a repulsive torque difference that existed at thetime when overriding occurred;

FIG. 18 is a system diagram of a vehicle driving assist system inaccordance with a fourth embodiment of the present invention;

FIG. 19 is a series of diagrams (a) to (d) showing the operation of avehicle driving assist system in accordance with the fourth embodimentof the present invention;

FIG. 20 is a flowchart showing the processing steps of a vehicle drivingassistance control program in accordance with the fourth embodiment ofthe present invention;

FIG. 21 is a flowchart for explaining the processing steps executed inorder to calculate a modified reaction force;

FIG. 22 is a graph plotting a prescribed amount of time for executing areaction force modification control A versus the accelerator pedaldepression rate;

FIG. 23 is a graph plotting a modified reaction force command valueversus the reaction force command value;

FIG. 24 is a pair of graphs (a) and (b) plotting the operational actionof the fourth embodiment of the present invention;

FIG. 25 is a graph plotting the modified reaction force command valueversus the risk potential RP in accordance with a first variation of thefourth embodiment of the present invention;

FIG. 26 is a pair of graphs (a) and (b) plotting the modified reactionforce command value change amount versus the reaction force commandvalue change amount for the reaction force modification controls A andB, respectively, in accordance with a second variation of the fourthembodiment of the present invention;

FIG. 27 is a plot illustrating the operational action of the secondvariation of the fourth embodiment of the present invention;

FIG. 28 is a plot illustrating the operational action of a thirdvariation of the fourth embodiment of the present invention;

FIG. 29 is a system diagram of a vehicle driving assist system inaccordance with a fifth embodiment of the present invention;

FIG. 30 is a flowchart showing the processing steps of a vehicle drivingassistance control program in accordance with the fifth embodiment ofthe present invention;

FIG. 31 is a flowchart for explaining the processing steps executed inorder to calculate a modified reaction force in accordance with acontrol 1 of the present invention;

FIG. 32 is a flowchart for explaining the processing steps executed inorder to calculate a modified reaction force in accordance with acontrol 2 of the present invention;

FIG. 33 is a flowchart for explaining the processing steps executed inorder to calculate a modified reaction force in accordance with acontrol 3 of the present invention;

FIG. 34 is a flowchart showing the processing steps of a vehicle drivingassistance control program in accordance with a sixth embodiment of thepresent invention;

FIG. 35 is a flowchart for explaining the processing steps executed inorder to calculate a modified reaction force in accordance with a passsituation;

FIG. 36 is a flowchart for explaining the processing steps executed inorder to calculate a modified reaction force in accordance with anapproach situation;

FIG. 37 is a flowchart showing the processing steps of a vehicle drivingassistance control program in accordance with a seventh embodiment ofthe present invention;

FIG. 38 is a flowchart for explaining the processing steps executed inorder to calculate a modified reaction force in accordance with acontrol 2′ of the present invention;

FIG. 39 is a flowchart for explaining the processing steps executed inorder to calculate a modified reaction force in accordance with acontrol 3′ of the present invention;

FIG. 40 is a flowchart showing the processing steps of a vehicle drivingassistance control program in accordance with the eighth embodiment ofthe present invention; and

FIG. 41 is a flowchart for showing the processing steps executed inorder to calculate a modified risk potential value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Selected embodiments of the present invention will now be explained withreference to the drawings. It will be apparent to those skilled in theart from this disclosure that the following descriptions of theembodiments of the present invention are provided for illustration onlyand not for the purpose of limiting the invention as defined by theappended claims and their equivalents.

Referring initially to FIG. 1, an exemplarily system diagram of avehicle driving assist system is illustrated in accordance with thepresent invention. FIG. 2 is a schematic perspective view of a vehicle(hereinafter also called “the host vehicle”) in which the vehicledriving assist system shown in FIG. 1 is installed in accordance withthe present invention. With the present invention, as explained below,it possible to prevent transition between a first driving assistancecontrol system and a second driving assistance control system fromimpeding a driver's ability to operate a vehicle in accordance with hisor her intent by adjusting the controls executed by the vehicle drivingassist system in such a manner that a smooth control transition isachieved.

The main structures and features of the vehicle driving assist systemwill now be explained. In order to detect the running conditions of thehost vehicle, the vehicle driving assist system includes, among otherthings, a laser radar device 10 that serves as a headway distancesensor. The laser radar device 10 is mounted to a front grill portion, abumper portion, or the like of the host vehicle. The laser radar device10 horizontally scans a region in front of the host vehicle with aninfrared light pulse. The laser radar device 10 then measures thereflected light resulting from the infrared light reflecting off of aplurality of reflecting objects located in front of the host vehicle(normally, the rear ends of preceding vehicles). The region in front ofthe host vehicle scanned by the laser radar device 10 is, for example,±6 degrees with respect to the front of the host vehicle and the systemdetects preceding objects existing within this angular range. Bymeasuring the time required for the reflected light to arrive, the laserradar device 10 detects the headway or following distance with respectto the preceding vehicle(s) or other obstacle(s). The detected headwaydistances and relative velocities are sent to a controller 150.

The vehicle speed sensor 20 detects the speed of the host vehicle inwhich the system 1 is installed by measuring the rotational speed of thewheels or the rotational speed of the output side of the transmission.The vehicle speed sensor 20 outputs the detected vehicle speed to thecontroller 150.

A frontward camera 30 comprising, for example, a small CCD camera orCMOS camera mounted on an upper portion of the front windshield servesto capture an image of the circumstances of a region of road in front ofthe host vehicle. The frontward camera 30 sends a signal of the capturedimage an image processing device 40, which processes the image and sendsthe image to the controller 150. The detection region of the frontwardcamera 30 is a region within ±30 degrees horizontally with respect tothe longitudinal centerline of the host vehicle. The frontward camera 30captures an image of the forward road situation within this detectionregion

As shown in FIG. 3, a servomotor 80 and an accelerator pedal strokesensor 90 are connected to an accelerator pedal 71 through a linkmechanism. The accelerator pedal stroke sensor 90 detects the strokeamount (actuation amount) Ap of the accelerator pedal 71 as a rotationalangle of the servomotor 80; depression of the accelerator pedal 71 isconverted into a rotational angle of the servomotor 80 by the linkagemechanism.

A steering switch unit 100 is installed, for example, on a steeringwheel 105 such that it is easy for a driver to operate. The steeringswitch unit 100 includes hand-operated switches for turning on and off afollowing distance control and a risk potential conveyance control (bothdescribed later). The steering switch unit 100 also includeshand-operated switches for setting a target time to headway for useduring the following distance control. When a driver operates any of theswitches, a signal corresponding to the switch operation is sent fromthe steering switch unit 100 to the controller 150.

The controller 150 comprises a CPU and a ROM, a RAM, and othercomponents peripheral to the CPU and serves to control the entirevehicle driving assist system 1. The controller 150 comprises thefollowing units provided, for example, in a CPU software format: a riskpotential calculating unit 151, a pedal reaction force command valuecalculating unit 152, a first target deceleration rate calculating unit153, a second target deceleration rate calculating unit 154, a modifiedtarget deceleration rate calculating unit 155, and a driving assistancecontrol selecting unit 156.

The risk potential calculating unit 151 calculates a risk potential RPindicating the degree of convergence of the host vehicle with respect toan obstacle existing in front of the host vehicle. The risk potential RPis calculated based on the vehicle speed detected by the vehicle speedsensor 20, the relative velocity and the headway distance between thehost vehicle and the obstacle in front of the host vehicle detected bythe laser radar 10, and the image data of the vicinity of the hostvehicle outputted from the image processing device 40. Based on the riskpotential RP calculated by the risk potential calculating unit 151, thepedal reaction force calculating unit 152 calculates a command value FAfor the actuation reaction force to be exerted by the accelerator pedal71. The first target deceleration rate calculating unit 153 serves tocalculate a target deceleration rate to be imposed on the host vehicleduring risk potential conveyance control (hereinafter called “RPconveyance control”); the calculation is based on the risk potential RP.In this embodiment, the risk potential is conveyed to the driver bycontrolling the braking/driving force exerted against the host vehicle.

The second target deceleration rate calculating unit 154 serves tocalculate a target deceleration rate to be used during the followingdistance control. The following distance control is contrived to controlthe headway distance and the vehicle speed such that the host vehiclemaintains a certain relative running condition with respect to apreceding obstacle. In other words, the following distance controlcontrols the acceleration and deceleration of the host vehicle such thatthe headway distance between the host vehicle and a preceding vehicle isheld substantially constant while also imposing a preset vehicle speedas an upper limit of the vehicle speed. The second target decelerationrate calculating unit 154 calculates the target deceleration rate forfollowing the preceding vehicle based on a target time to headway inputfrom the steering switch unit 100.

The modified target deceleration rate calculating unit 155 serves tocalculate a modified target deceleration rate for use when there is atransition in the operating states of the RP conveyance control and thefollowing distance control. The calculation is based on the targetdeceleration rate calculated by the first target deceleration ratecalculating unit 153 and the target deceleration rate calculated by thesecond target deceleration rate calculating unit 154. The drivingassistance control selecting unit 156 selects an operating state foreach of the RP conveyance control and the following distance controlbased on the target deceleration rates and the modified targetdeceleration rate and outputs operation commands to an engine controller50, a brake actuator 60, and a display device 110.

An accelerator pedal reaction force control device 70 controls theactuation reaction force exerted by the accelerator pedal 71 based on acommand value received from the controller 150. The servomotor 80controls the actuation reaction force generated when the driver operatesthe accelerator pedal 71 by controlling its torque and rotational anglebased on a command value from the accelerator pedal reaction forcecontrol device 70. The normal reaction force characteristic exhibited bythe accelerator pedal 71 when the accelerator pedal reaction forcecontrol is not executed is set such that, for example, the acceleratorpedal reaction force increases linearly as the accelerator pedalactuation amount Ap increases. The normal accelerator pedal reactionforce characteristic can be realized by utilizing the spring force of atorsion spring (not shown) provided at the rotational center of theaccelerator pedal 71.

The engine controller 50 calculates a control command to be sent to theengine and serves as a driving force control means for controlling thedriving force exerted against the host vehicle. The engine controller 50controls the driving force in such a manner as to achieve the targetdeceleration rate received from the controller 150. More specifically,the engine controller 50 uses a relationship like that shown in FIG. 4to calculate the driver's requested driving force drv_trq based on theaccelerator pedal actuation amount SA. Then, the engine controller 50calculates the control command to be sent to the engine by subtracting avalue equivalent to the target deceleration rate from the driver'srequested driving force drv_trq. During the following distance control,the acceleration of the host vehicle is controlled in such a manner asto achieve the set target time to headway while ignoring acceleratorpedal actuation amount SA.

The brake actuator 60 serves as a braking force control section thatcontrols a braking force exerted against the host vehicle by outputtinga brake fluid pressure command. The brake actuator 60 controls thebraking force in such a manner as to achieve the target decelerationrate received from the controller 150. Brake devices provided on thewheels of the host vehicle operate in accordance with the command issuedfrom the braking actuator 60.

The display device 110 is, for example, a dot matrix display. As shownin FIG. 5, the display device 110 is arranged in a portion of acombination meter 111 provided in the instrument panel in front of thedriver's seat so as to be readily viewable by the driver. The displaydevice 110 serves to display the operating state of the currentlyexecuted control in accordance with a command from the controller 150.

The operation of a vehicle driving assist system 1 in accordance withthe first embodiment will now be explained. First, an overview of theoperation will be provided.

The controller 150 calculates the risk potential RP of the host vehiclewith respect to an obstacle in the vicinity of the host vehicle based onthe running condition of the host vehicle and the traveling environment(traveling situation) surrounding the host vehicle detected by the laserradar 10, the vehicle speed sensor 20, and the frontward camera 30. Theterm “risk potential RP” refers to the degree of risk or possibility ofdanger. In this embodiment, the risk potential is contrived to increaseas the host vehicle and an obstacle existing in the vicinity of the hostvehicle approaches each other. Thus, it can be the that the riskpotential is a physical quantity that expresses how close the hostvehicle and the obstacle are to each other, i.e., the degree to whichthe host vehicle and the obstacle have drawn near to each other (degreeof convergence).

The controller 150 controls the actuation reaction force exerted by theaccelerator pedal 71 based on the risk potential RP and convey the riskpotential RP to the driver by causing the host vehicle to decelerate,thereby alerting the driver (RP conveyance control). The controller 150is also configured to control the acceleration/deceleration of the hostvehicle based on a target time to headway set by the driver such that asubstantially constant distance is maintained between the host vehicleand a preceding vehicle (following distance control).

Thus, the controller 150 can execute a plurality of different controlsthat control the deceleration of the host vehicle. The RP conveyancecontrol and the following distance control are turned “on” and “off” byoperating respective switches of the steering switch unit 100. FIG. 6shows the operating modes of the RP conveyance control and the followingdistance control. A state in which the RP conveyance control and thefollowing distance control are both not operating (OFF) is called Mode 0and a state in which the RP conveyance control is operating (ON) and thefollowing distance control is not operating is called Mode 1.

A state in which the RP conveyance control is not operating and thefollowing distance control is operating is called Mode 2. If the driverdepresses the accelerator pedal 71 while the vehicle driving assistsystem 1 is in Mode 2, the following distance control is overridden andthe vehicle driving assist system 1 shifts to Mode 0, in which neitherthe following distance control nor the RP conveyance control isoperating. When the accelerator pedal 71 is released, the override state(state in which following distance control is overridden) is cancelledand the mode returns to Mode 2.

A state in which both the RP conveyance control and the followingdistance control are operating is called Mode 3. In Mode 3, operation ofthe following distance control is given priority over operation of theRP conveyance control. In other words, Mode 3 is a state in which bothcontrols can operate, but only the following distance control isactually operating. If the driver depresses the accelerator pedal 71while the vehicle driving assist system 1 is in Mode 3, then thefollowing distance control is overridden and the vehicle driving assistsystem 1 shifts to Mode 4, in which only the RP conveyance control isoperating. When the accelerator pedal 71 is released, the override stateis cancelled and the mode returns to Mode 3.

Thus, when the mode shifts from Mode 3 (in which both RP conveyancecontrol and following distance control can be executed) to Mode 4, thecontrol that is actually executed is switched from the followingdistance control to the RP conveyance control. When the system switchesfrom following distance control to RP conveyance control in this manner,there is a possibility that the deceleration rate imposed on the hostvehicle will be increase due to the difference between the targetdeceleration rate set during the following distance control and the newtarget deceleration rate set for the RP conveyance control. In such acase, the behavior of the host vehicle may deviate greatly from thedriver's intent because the driver is depressing the accelerator pedal71 and attempting to accelerate. When the override state is cancelledand the mode returns from Mode 4 to Mode 3, there is a possibility thatthe deceleration rate will decrease in comparison with the decelerationrate imposed during RP conveyance control.

Therefore, in the first embodiment, a control modification is executedwhen the vehicle driving assist system 1 switches between the followingdistance control and the RP conveyance control in order to prevent thevehicle behavior from being contrary to the intent of the driver.

The operation of the vehicle driving assist system 1 in accordance withthe first embodiment will now be explained in more detail with referenceto FIGS. 7 and 8. FIGS. 7 and 8 show a flowchart of the processing stepsof a driving assistance control program executed by the controller 150.This control loop is executed continuously once per prescribed timeperiod, e.g., every 50 msec.

In step S101, the controller 150 detects an obstacle existing in frontof the vehicle and the headway distance D between the vehicle and thepreceding obstacle based on signals from the laser radar 10 and theimage processing device 40. Here, it is assumed, for example, that thepreceding obstacle is a preceding vehicle. In step S103, the controller150 detects the preceding vehicle speed V2 of the preceding vehicledetected in step S101. The preceding vehicle speed V2 can be detectedusing, for example, intervehicle communication or a calculation based oninput signals from the laser radar 10 and image processing device 40. Instep S105, the controller 150 detects the host vehicle speed V1 via thevehicle speed sensor 20.

In step S107, the controller 150 reads an operation signal from thesteering switch unit 100. In step S109, the controller 150 determinesthe current operating states of the RP conveyance control and followingdistance control based on the running condition and travelingenvironment of the vehicle read in steps S101 to S105 and the operationsignal of the steering switch unit 100 read in step S107. In short, thecontroller 150 determines if each of the RP conveyance control and thefollowing distance control is operating or not operating.

In step S111, the controller 150 determines if the RP conveyance controlis operating. If RP conveyance control is operating, the controller 150proceeds to step S113. If not, the controller 150 proceeds to step S125.In step S113, the risk potential RP of the vehicle with respect to thepreceding object is calculated based on the running condition and thetraveling environment of the vehicle detected in steps S101 to S105. Themethod of calculating the risk potential RP will now be described.

Consider a model in which it is assumed that an imaginary elastic body170 is provided on the front of the vehicle 160 in which the drivingassistance system 1 is installed, as shown in diagram (a) of FIG. 9. Theimaginary elastic body 170 touches against the preceding vehicle 180 andis compressed, thereby generating a pseudo traveling resistance againstthe movement of the vehicle 160. The risk potential RP with respect tothe obstacle is defined to be the repulsive force that results when, asshown in diagram (b) of FIG. 9, the imaginary elastic body 170 contactsthe preceding vehicle 180 and is compressed. In this embodiment, therepulsive force of an imaginary elastic body correlated to the time tocollision TTC between the vehicle and the preceding obstacle and therepulsive force of an imaginary elastic body correlated to the time toheadway THW between the vehicle and the preceding obstacle are bothcalculated and the larger of the two calculated repulsive forces isselected in order to determine the risk potential RP. The method ofcalculating the risk potential RP will now be explained with referenceto the flowchart of FIG. 10.

In step S1301, the controller 150 calculates the time to headway THW andthe time to collision TTC between the vehicle and the precedingobstacle. The time to headway THW is a physical quantity indicating thetime required for the vehicle to reach the current position of thepreceding object, e.g., preceding vehicle, and is calculated usingEquation 1 below based on the host vehicle speed V1 and the headwaydistance D.THW=D/V1  (Equation 1)

The time to collision TTC is a physical quantity indicating the currentdegree of convergence of the vehicle with respect to the precedingvehicle. More specifically, the time to collision TTC is a valueindicating the number of seconds until the headway distance D becomeszero and the vehicle contacts the preceding vehicle if the currentrunnning condition continues, i.e., if the host vehicle speed V1 and therelative velocity Vr remain constant. The relative velocity Vr (i.e.,V1=V1−V2) is treated as zero (Vr=0) when the traveling speed of thepreceding vehicle is larger than the traveling speed of the vehicle. Thetime to collision TTC with respect to the preceding obstacle is foundusing the Equation 2 shown below.TTC=D/Vr  (Equation 2)

The smaller the time to collision TTC is, the more eminent the contactwith the preceding vehicle is and the larger the degree of convergencewith respect to the preceding vehicle is. For example, it is known thatwhen approaching a preceding vehicle, most drivers start taking actionto decelerate before the time to collision TTC reaches 4 seconds orless.

In step S1302, the controller 150 compares the time to headway THW to athreshold value TH1. The threshold value TH1 is set to an appropriatetime to headway value (e.g., 2 seconds) for determining that it is timefor control to be started. If the time to headway THW is smaller thanthe threshold value TH1 (THW<TH1), the controller 150 proceeds to stepS1303. In step S1303, the controller 150 calculates a risk potentialRPthw based on the time to headway THW by using the host vehicle speedV1 and the time to headway THW in the Equation 3 shown below.RPthw=K_THW×(TH1−THW)×V1  (Equation 3)

In Equation 3, the term K_THW is a spring constant of the imaginaryelastic body correlated to the time to headway THW and the value ofTH1×V1 corresponds to the length of the imaginary elastic body.

If the time to headway THW is found to be equal to or larger thanthreshold value TH1 (THW≧TH_THW) in step S1302, the controller 150proceeds to step S1304 and sets the value of the risk potential RPthw to0.

In step S1305, the controller 150 compares the time to collision TTC toa threshold value TH2. The threshold value TH2 is set to an appropriatetime to collision value (e.g., 8 seconds) for determining that it istime for control to be started. If the time to collision TTC is smallerthan the threshold value TH2 (TCC<TH2), then the controller 150 proceedsto step S1306. In step S1306, the controller 150 calculates a riskpotential RPttc based on the time to collision TTC by using the relativevelocity Vr and the time to collision TTC in the Equation 4 shown below.RPttc=K_TTC×(TH2−TTC)×Vr  (Equation 4)

In Equation 4, the term K_TTC is the spring constant of the imaginaryelastic body correlated to the time to collision TTC and the value ofTH2×Vr corresponds to the length of the imaginary elastic body.

If the time to collision TTC is found to be equal to or larger than TH2(TTC≧TH2) in step S1305, then the controller 150 proceeds to step S1307and sets the value of the risk potential RPttc to 0.

In step S1308, the controller 150 compares the risk potential RPthwcalculated based on the time to headway THW in step S1303 or S1304 tothe risk potential RPtcc calculated based on the time to collision TTCin step S1306 or S1307 and selects the larger of the two values to beused as the final risk potential RP.

After it calculates the risk potential RP in step S113, the controller150 proceeds to step S115. In step S115, the controller 150 calculatesan accelerator pedal reaction force command value FA based on the riskpotential RP calculated in step S113.

FIG. 11 is a graph plotting the reaction force command value FA versusthe risk potential RP. As shown in FIG. 11, the reaction force commandvalue FA is set to 0 when the risk potential RP is equal to or smallerthan a minimum value RPmin. This is done to prevent increases in theactuation reaction force of the accelerator pedal 71 from occurring whenthe risk potential RP in the vicinity of the vehicle is extremely smalland, thus, becoming an annoyance to the driver. The minimum value RP isset in advance to an appropriate value.

In the region where the risk potential RP exceeds the minimum valueRPmin, the reaction force command value FA increases exponentially withrespect to the risk potential RP. The reaction force command value FA isfound using the Equation 5 shown below.FA=α×RP^(n)  (Equation 5)

The constants α and n vary depending on the vehicle type and are set inadvance based on the results of driving simulations or practicalexperiments to appropriate values for effectively converting the riskpotential RP into a reaction force command value FA.

In step S117, the controller 150 calculates a repulsive torque rf1 to beexerted against the vehicle during RP conveyance control based on therisk potential RP calculated in step S113. The repulsive torque rf1corresponds to the repulsive force exerted against the vehicle 160 bythe imaginary elastic body 170 shown in diagrams (a) and (b) in FIG. 9and serves as a braking/driving force modification amount for loweringthe driving force of the vehicle or increasing the braking force of thevehicle during RP conveyance control. FIG. 12 is a graph plotting therepulsive torque rf1 versus the risk potential RP. As shown in FIG. 12,the repulsive torque rf1 increases gradually as the risk potential RPincreases beyond a prescribed value RP1.

In step S119, the controller 150 reads the accelerator pedal actuationamount Ap detected by the accelerator pedal stroke sensor 90 andcalculates a driver's requested driving force drv_trq based on theaccelerator pedal actuation amount Ap. The controller 150, similarly tothe engine controller 50, stores a map of the driver's requested drivingforce drv_trq versus the accelerator pedal actuation amount Ap like thatshown in FIG. 4 and calculates the driver's requested driving forcedrv_trq corresponding to the accelerator pedal actuation amount Ap basedon the map.

In step S121, the controller 150 determines if the following distancecontrol is operating. If the following distance control is operating,then the controller 150 proceeds to step S133 of FIG. 8. If not, thecontroller 150 proceeds to step S123. In a step S133, the controller 150determines if an override flag ow serving to indicate whether or not theoverride state exists is on. If the override flag ow is on, then thefollowing distance control is already overridden and the controller 150proceeds to step S141. If the override flag ow is off, then thecontroller 150 proceeds to step S135. In step S135, the controller 150determines if the accelerator pedal actuation amount Ap is larger than aprescribed value Apo. The prescribed value Apo is set to an acceleratorpedal actuation amount Ap substantially equivalent to a target vehiclespeed with which the target time to headway of the following distancecontrol can be achieved.

As previously explained, the following distance control is givenpriority when both the RP conveyance control and the following distancecontrol are operating, but the following distance control is overriddenand the system switches to RP conveyance control when the conditionAp>Apo occurs. If the condition Ap>Apo exists, then the controller 150proceeds to step S137. If the condition Ap≦Apo exists, then thecontroller 150 proceeds to step S147. In step S137, the controller 150calculates the repulsive torque rf2 o exerted by the following distancecontrol at the time when overriding started. The “time when overridingstarted” refers to the point in time when following distance control wasoverridden due to the accelerator pedal 71 exceeding the prescribedvalue Apo. The repulsive torque rf2 o serves as a braking/driving forcemodification amount used to modify the driving force or the brakingforce in order to achieve the target time to headway during followingdistance control. The repulsive torque rf2 o exerted by the followingdistance control at the time when overriding started is calculated usingthe Equation 6 below.rf2o=1/K2×dg2o  (Equation 6)

In Equation 6, dg2 o is the target deceleration rate imposed by thefollowing distance control at the time when overriding started and K2 isa coefficient for converting the repulsive torque into a targetdeceleration rate. Here, the inverse of the coefficient K2 is used toconvert the target deceleration rate dg2 o into the repulsive torque rf2o. The target deceleration rate dg2 o of the following distance controlis calculated using a well-known method in such a manner as to achievethe aforementioned target headway distance. In step S139, the controller150 changes the override flag ow to the ON state (turns it on) andproceeds to step S141.

In step S141, the controller 150 determines if a condition for cancelingthe braking/driving force modification control executed during theoverride state is satisfied. Examples of conditions for canceling thebraking/driving force modification control during the override state arelisted below.

Condition 1→The preceding vehicle that was detected at the time whenoverriding started is no longer detected.

Condition 2→A prescribed amount of time T has elapsed since overridingstarted.

Condition 3→The repulsive torque rf1 exerted by the RP conveyancecontrol during the override state becomes smaller than the repulsivetorque rf2 o exerted by the following distance control.

If none of the conditions 1 to 3 listed above is satisfied, then thecontroller 150 determines that braking/driving force control will beexecuted during the override state and proceeds to step S143. If any oneof the conditions 1 to 3 is satisfied, then the controller 150 proceedsto step S147. In step S143, the controller 150 sets the vehicletraveling assistance system 1 into Mode 4, in which the followingdistance control is overridden and only the RP conveyance control isexecuted.

In step S145, the controller 150 calculates a target deceleration ratedg to be used while the following distance control is overridden andonly RP conveyance control is executed. First, the controller 150compares the repulsive torque rf1 of the RP conveyance controlcalculated in step S117 to the repulsive torque rf2 o of the followingdistance control at the time when overriding started (calculated in stepS137). The controller 150 then calculates a modified braking/drivingforce rfc by subtracting the driver's requested driving force drv_trqfrom the smaller of the repulsive torques rf1 and rf2 o, as shown inEquation 7 below.rfc=min{rf1,rf2o}−drv _(—) trq  (Equation 7)

The target deceleration rate dg can be calculated by multiplying themodified braking/driving force rfc by the coefficient K2, as shown inFIG. 8.dg=K2×rfc  (Equation 8)

The controller proceeds from step S135 to step S147 if it determines instep S135 that the actuation amount Ap is equal to or smaller than theprescribed value Apo (Ap≦Apo) and from step S141 to step S147 if itdetermines in step S141 that one of the conditions for cancelling thebraking/driving force modification control executed during the overridestate is satisfied. In step S147, the controller 150 sets the vehicledriving assist system 1 to Mode 3, in which execution of followingdistance control is given priority over execution of RP conveyancecontrol. In step S149, the controller 150 turns the override flag ow offIn step S151, the controller 150 calculates a targetacceleration/deceleration rate dg2 for following distance control.Similarly to the target deceleration rate dg2 o corresponding to whenoverriding started, the target acceleration/deceleration rate dg2 iscalculated using a well-known method and calculated such that the settarget headway distance can be achieved.

If it determines in step S111 that RP conveyance control is notoperating, then the controller 150 proceeds to step S125 and determinesif following distance control is operating. If following distancecontrol is operating, then the controller 150 proceeds to step S127. Ifnot, then the controller 150 proceeds to step S131. In step S127, thecontroller 150 determines if the accelerator pedal actuation amount Apis larger than a prescribed value Apo. If the condition Ap≦Apo exists,then the controller 150 proceeds to step S129. If the condition Ap>Apoexists, the controller 150 proceeds to step S131.

In step S129, the controller 150 sets the vehicle driving assist systemto Mode 2, in which only following distance control is executed. In stepS131, the controller 150 sets the vehicle driving assist system to Mode0, in which the following distance control is overridden and neitherfollowing distance control nor RP conveyance control is executed. If itsets Mode 0, then the controller 150 proceeds to step S161 (explainedlater) without calculating a target deceleration rate (targetacceleration/deceleration rate).

If it goes to step S129 and sets Mode 2, then the controller 150proceeds to step S149 and sets the override flag ow to OFF. Then, instep S151, the controller 150 calculates a targetacceleration/deceleration rate dg2 for following distance control.Meanwhile, if it determines in step S121 that following distance controlis operating, then the controller 150 proceeds to step S123. In stepS123, the controller 123 sets the vehicle driving assist system 1 toMode 1, in which only RP conveyance control is executed. Then thecontroller 150 proceeds to step S153 and calculates a targetdeceleration rate dg1 for the RP conveyance control. The targetdeceleration rate dg1 is calculated using the repulsive torque rf1calculated in step S117 in the Equation 9 shown below.dg1=K2×rf1  (Equation 9)

After calculating the target deceleration rate (targetacceleration/deceleration rate) for Mode 1, 2, or 3, the controller 150proceeds to step S155. In step S155, the controller 150 sends theaccelerator pedal reaction force command value FA calculated in stepS115 to the accelerator pedal reaction force control device 70. Theaccelerator pedal reaction force control device 70 controls theservomotor 80 based on the command from the controller 150 and therebycontrols the actuation reaction force that acts when the driver operatesthe accelerator pedal 71.

In step S157, the controller 150 sends the target deceleration rate dgor dg1 or the target acceleration/deceleration rate dg2 calculated instep S145, S151, or S153 to the engine controller 50. The enginecontroller 50 compares the target deceleration rate (targetacceleration/deceleration rate) to the driver's requested driving forcedrv_trq determined based on the accelerator pedal actuation amount Apand modifies the driver's requested driving force drv_trq to a smallervalue suitable for achieving the target deceleration rate (targetacceleration/deceleration rate). The modified driver's requested drivingforce value is outputted as an engine control command. Thus, the drivingforce exerted against the vehicle is reduced. If the driving forcereduction amount corresponding to the target deceleration rate(acceleration/deceleration rate) is larger than the driver's requesteddriving force drv_trq, in step S159 the controller 150 executes abraking force control serving to increase the braking force.

In step S159, the controller 150 sends the target deceleration rate dgor dg1 or the target acceleration/deceleration rate dg2 calculated instep S145, S151, or S153 to the brake actuator 60. Braking force controlis executed when the driving force reduction amount corresponding to thetarget deceleration rate (target acceleration/deceleration rate) islarger than the driver's requested driving force drv_trq and the targetdeceleration rate (target acceleration/deceleration rate) cannot beachieved with driving force control alone. More specifically, in orderto compensate for the amount by which the deceleration achieved withdriving force control does not satisfy the target deceleration rate(target acceleration/deceleration rate), the controller 150 issues abrake fluid pressure command contrived to modify a driver's requestedbraking force based on the brake pedal actuation amount to a largervalue. As a result, the braking force exerted against the vehicle isincreased. Thus, by reducing the driving force acting against thevehicle and increasing the braking force acting against the vehicle, theoverall state of the vehicle is controlled such that the targetdeceleration rate (target acceleration/deceleration rate) is achieved.

In step S161, the controller 150 sends a signal to the display device110 instructing the display device to indicate the operating states ofthe RP conveyance control and the following distance control. Diagrams(a) and (b) of FIGS. 13( a) and (b) show examples of what is displayedon the display device 110 to indicate the operating state of the vehicledriving assist system 1. Diagram (a) of FIG. 13 shows an example of whatis displayed during Mode 1 or Mode 4 when RP conveyance control isoperating. Diagram (b) of FIG. 13 shows an example of what is displayedduring Mode 2 or Mode 3 when following distance control is operating. Ifa target vehicle speed is set for following distance control, the targetvehicle speed is also displayed. After the display signal is sent, thecurrent cycle of the control loop ends.

The first embodiment described heretofore can thus provide the followingoperational effects.

(1) The vehicle driving assist system 1 detects an obstacle existing infront of the vehicle in which the system 1 is installed and calculate arisk potential RP of the vehicle with respect to the obstacle based onthe obstacle detection results. Then, based on the calculated riskpotential RP, the system 1 executes an RP conveyance control contrivedto control the actuation reaction force exerted by a driver-operateddriving operation device and/or a braking/driving force exerted againstthe vehicle and a following distance control contrived to control thebraking/driving force of the vehicle such that a headway distance D ismaintained between the vehicle and the obstacle. The vehicle drivingassist system 1 detects transitions of the operating states of the RPconveyance control and the following distance control and adjusts thecontrols when it detects a transition of operation state. By adjustingthe control when the operating state of one or both of the two differentcontrols changes, the control transition can be accomplished smoothlywithout impeding the driver's ability to drive (operate) the vehicle inaccordance with his or her intent.

(2) The following distance control is given priority when the vehicledriving assist system 1 is in a state in which both the RP conveyancecontrol and the following distance control can operate. Mode 3 (shown inFIG. 6) corresponds to the state in which both controls can operate butfollowing distance control is given priority. By executing followingdistance control, the driving operations performed by the driver can beassisted and the burden born by the driver can be lightened.

(3) When following distance control is overridden, RP conveyance controlis executed. The transition from Mode 3 to Mode 4 (shown in FIG. 6)corresponds a situation in which following distance control isoverridden. Since the control to which priority is given, i.e., eitherRP conveyance control or following distance control, changes based onsuch factors as the running condition of the vehicle and operationsperformed by the driver, the control can be optimized to the conditionsexisting at a particular time.

(4) The controller 150 detects transitions in the operating states ofthe RP conveyance control and following distance control based on thestates of switches operated by the driver. As a result, a controlcorresponding to the intent of the driver can be executed.

(5) In addition to detecting transitions based on switch operations, thecontroller 150 detects transitions in the operating states of the RPconveyance control and following distance control based on the actuationstate of a driver-operated driving operation device. As a result, acontrol that accommodates the driving operations intended by the drivercan be executed. The driving operation device is, for example, anaccelerator pedal 71. It is also possible to detect a control transitionbased on the actuation state of a brake pedal instead of an acceleratorpedal 71.

(6) When it detects a transition from a state in which the followingdistance control was operating to a state in which the RP conveyancecontrol is operating, the controller 150 modifies the braking/drivingforce controlled by the RP conveyance control. More specifically, thecontroller 150 executes control contrived to modify the braking/drivingforce when the mode changes from Mode 3 to Mode 4. Thus, by modifyingthe braking/driving force when the system 1 switches between the twodifferent controls, the control transition can be accomplished smoothlywithout impeding the driver's ability to drive (operate) the vehicle inaccordance with his or her intent.

(7) When the controller 150 modifies the braking/driving forcecontrolled by the RP conveyance control, it bases the modification onthe target deceleration rate dg2 o that was being used by the followingdistance control at the point in time of the transition, the targetdeceleration rate of the RP conveyance control, and the acceleratorpedal actuation amount Ap. As a result, when the system 1 switches fromfollowing distance control to RP conveyance control, the controltransition can be accomplished smoothly without impeding the driver'sability to drive (operate) the vehicle in accordance with his or herintent.

(8) More specifically, the target deceleration rate dg used in order tomodify the braking/driving force is calculated by selecting the smallerof a braking/driving force modification amount (e.g., repulsive torquerf2 o) corresponding to the target deceleration rate dg2 o that was usedby the following distance control at the point in time of the transitionand a braking/driving force modification amount (e.g., repulsive torquerf1) corresponding to the target deceleration rate of the RP conveyancecontrol and subtracting from the selected value the driver's requesteddriving force drv_trq determined based on the accelerator pedalactuation amount Ap. As a result, the control transition can beaccomplished smoothly without impeding the driver's ability to drive(operate) the vehicle in accordance with his or her intent. Morespecifically, when the following distance control is overridden duedepression of the accelerator pedal 71 by the driver, a decelerationrate that is larger than the deceleration rate that was imposed by thefollowing distance control does not occur. Furthermore, since thedriver's requested driving force drv_trq increases as the amount bywhich the accelerator pedal 71 is depressed increases, the targetdeceleration rate will decrease during braking/driving forcemodification control executed when the following distance control isoverridden. As a result, a system that reflects the intent of the driverregarding acceleration can be realized.

(9) The controller 150 stops modifying the braking/driving force when(a) the obstacle with respect to which the risk potential RP wascalculated at the point in time of the transition is no longer detected,(b) the braking/driving force modification amount corresponding to thetarget deceleration rate of the RP conveyance control becomes smallerthan the braking/driving force modification amount corresponding to thetarget deceleration rate dg2 o of the following distance control at thepoint in time of the transition, or (c) a prescribed amount of time Thas elapsed since the time of the transition. As a result, a smoothtransition from following distance control to RP conveyance control canbe accomplished.

In the first embodiment, a “transition in the operating states of the RPconveyance control and the following distance control” means switchingbetween RP conveyance control and following distance control andincludes the point in time when the switch actually occurred as well astimes occurring slightly before or after the point in time. Thus, thetransition should be thought of as a transition process. Meanwhile, “atthe point in time of the transition” means the point in time when aswitch between RP conveyance control and following distance controlactually occurred.

Second Embodiment

A vehicle driving assist system in accordance with a second embodimentof the present invention will now be explained. The basic constituentfeatures of a vehicle driving assist system 1 in accordance with thesecond embodiment are the same as those of the first embodiment shown inFIGS. 1 and 2. The second embodiment will be explained chiefly bydescribing its differences with respect to the first embodiment.

In the second embodiment, when the braking/driving force is modifiedwhile the following distance control is overridden, the driver'srequested driving force drv_trq is modified in a manner that takes intoaccount the accelerator pedal depression rate Apv during the overridestate.

The operation of a vehicle driving assist system 1 in accordance withthe second embodiment will now be explained in detail with reference toFIGS. 14 and 15. FIGS. 14 and 15 are flowcharts showing the processingsteps of a driving assistance control program executed by the controller150. This control loop is executed continuously once per prescribed timeperiod, e.g., every 50 msec. The control processing of the steps S101 toS143 and S145 to S161 is the same as in the flowchart shown in FIGS. 7and 8 and explanations of those steps are omitted for the sake ofbrevity.

After the system 1 is set to Mode 4 in step S143, the controller 150proceeds to step S201 and calculates the depression rate Apv of theaccelerator pedal 71 during the override state. The accelerator pedaldepression rate Apv can be calculated, for example, by finding aderivative of the accelerator pedal actuation amount Ap with respect totime. The controller 150 then calculates a driver's requested drivingforce modification coefficient Kapv based on the accelerator pedaldepression rate Apv. FIG. 16 is a graph plotting a coefficient f(apv)for calculating the driver's requested driving force modificationcoefficient Kapv versus the accelerator pedal depression rate Apv.

As shown in FIG. 16, the coefficient f(apv) is fixed at a value of 1 foraccelerator pedal depression rates Apv ranging from 0 to a prescribedvalue Apv1. Meanwhile, the coefficient f(apv) increases gradually as theaccelerator pedal depression rate Apv increases beyond the prescribedvalue Apv1. The driver's requested driving force modificationcoefficient Kapv=f(apv).

After it calculates the driver's requested driving force modifyingcoefficient Kapv in step S201, the controller 150 proceeds to step S145and calculates the target deceleration rate dg. First, the controller150 compares the repulsive torque rf1 of the RP conveyance controlcalculated in step S117 to the repulsive torque rf2 o calculated in stepS137 (the repulsive torque rf2 o is the repulsive torque imposed by thefollowing distance control at the time when overriding started). Thecontroller 150 then calculates a modified braking/driving force rfcusing the Equation 10 shown below based on the smaller of the repulsivetorques rf1 and rf2 o, the driver requested driving force dr_trq, andthe driver requested driving force modification coefficient Kapv.rfc=min{rf1,rf2o}−drv _(—) trq×Kapv  (Equation 10)

The target deceleration rate dg can be calculated by multiplying themodified braking/driving force rfc calculated with Equation 10 by thecoefficient K2.

In addition to the operational effects exhibited by the firstembodiment, the second embodiment also achieves the following additionaleffects.

(1) The controller 150 can calculate the target deceleration rate to beused for modifying the braking/driving force (braking/driving forcemodification control) in a manner that takes into account theaccelerator pedal depression rate Apv. As a result, the driver's intentwith respect to acceleration can be taken into account in thecalculation of the target deceleration rate.

(2) The controller 150 sets the driver requested driving force drv_trqsuch that the larger the accelerator pedal depression rate Aov is, thelarger value to which the driver requested driving force drv_trq is set.More specifically, as shown in FIG. 16, the coefficient Kapv that ismultiplied by the driver requested driving force drv_trq increases asthe accelerator pedal depression rate Apv increases. As a result, thefaster the driver depresses the accelerator pedal 71, the smaller thetarget deceleration rate becomes and, thus, the control is contrived toreflect the driver's intent regarding acceleration.

Third Embodiment

A vehicle driving assist system in accordance with a third embodiment ofthe present invention will now be explained. The basic constituentfeatures of a vehicle driving assist system 1 in accordance with thethird embodiment are the same as those of the first embodiment shown inFIGS. 1 and 2. The third embodiment will be explained chiefly bydescribing its differences with respect to the first embodiment.

In the third embodiment, the prescribed amount of time T for which thebraking/driving force modification control is continued during theoverride state (state in which following distance control is overridden)is modified based on the difference Δrf between the repulsive torque rf1of the RP conveyance control and the repulsive torque rf2 o of thefollowing distance control at the time when overriding of the followingdistance control started. The difference Δrf equals rf1 minus rf2 o(Δrf=rf1−rf2 o).

FIG. 17 shows a graph plotting a modification control continuation timecoefficient pt and the repulsive torque difference Δrf at the time whenoverriding started. As shown in FIG. 17, the coefficient pt is fixed ata value of 1 for differences Δrf ranging from 0 to a prescribed valueΔrf1. Meanwhile, the coefficient pt increases gradually as thedifference Δrf increases beyond the prescribed value Δrf1, i.e., as therepulsive torque rf1 of the RP conveyance control increases with respectto the repulsive torque rf2 o of the following distance control. Themodified time value Tc of the braking/driving force modification controlcontinuation time T is calculated using Equation 11 shown below.Tc=T×pt  (Equation 11)

If it determines in step S141 that the modified time Tc has elapsedsince overriding started, the controller 150 proceeds to step S147 andsets the system 1 to Mode 3, in which only following distance control isexecuted. Meanwhile, if the modified time Tc has not yet elapsed sinceoverriding started, and another condition is not satisfied, thecontroller 150 determines that the override state exists and proceeds tostep S143, where it sets the mode to Mode 4, i.e., RP conveyance controlonly.

In addition to the operational effects exhibited by the firstembodiment, the third embodiment also achieves the following additionaleffects.

The controller 150 sets the braking/driving force modification controlcontinuation time (prescribed amount of time) T in a variable fashionbased on the difference between a braking/driving force modificationamount corresponding to the target deceleration rate of the followingdistance control at the point in time of the transition and abraking/driving force modification amount corresponding to the targetdeceleration rate of the RP conveyance control. More specifically, asshown in FIG. 17, the coefficient pt of the braking/driving forcemodification control continuation time T is set such that it becomeslarger as the difference Δrf (=rf1−rf2 o) between the repulsive torquerf2 o of the following distance control at the point in time of thetransition and the repulsive torque rf1 of the RP conveyance controlincreases. As a result, since the braking/driving force modificationcontrol continuation time T increases as the difference Δrf increases, asmooth control transition can be accomplished.

Variation

In the first to third embodiments, the braking/driving force controlledby the RP conveyance control is modified when the following distancecontrol is overridden and the system 1 switches to RP conveyancecontrol. However, the invention is not limited to such an arrangement.It is also possible to configure the vehicle driving assist system suchthat when it detects a transition from a state in which the followingdistance control was operating to a state in which the RP conveyancecontrol is operating, the controller 150 modifies an actuation reactionforce controlled by the RP conveyance control. More specifically, asupplemental characteristic added to the accelerator pedal reactionforce by the RP conveyance control at the time when overriding startscan be set in a variable manner. For example, the supplementalcharacteristic can be set such that the slower the accelerator pedaldepression rate Apv is at the time when the overriding starts, the moregradually the accelerator pedal reaction force increases as theaccelerator pedal is depressed.

It is also possible to configure the vehicle driving assist system suchthat when it detects a transition from a state in which the followingdistance control was operating to a state in which the RP conveyancecontrol is operating, the controller 150 modifies the risk potential RPof the vehicle with respect to the obstacle.

Similarly to the embodiments, a smooth transition from followingdistance control to RP conveyance control can be achieved by modifyingthe actuation reaction force or the risk potential RP.

Fourth Embodiment

A vehicle driving assist system in accordance with a fourth embodimentof the present invention will now be explained. FIG. 18 is a systemdiagram of a vehicle driving assist system 2 in accordance with thefourth embodiment. In FIG. 18, parts having the same functions as theparts of the first embodiment shown in FIGS. 1 and 2 are indicated withthe same reference numerals. The fourth embodiment will be explainedchiefly by describing its differences with respect to the firstembodiment.

As shown in FIG. 18, a vehicle driving assist system 2 in accordancewith the fourth embodiment comprises a laser radar 10, a vehicle sensor20, a frontward camera 30, an image processing device 40, an acceleratorpedal stroke sensor 90, a steering switch unit 100, a controller 200, anaccelerator pedal reaction force control device 70, a servomotor 80, aninformation presentation controller 510, and an information presentingdevice 520. The information presenting device 520 comprises, forexample, an indicator lamp for RP conveyance control and an indicatorlamp for following distance control.

Similarly to the first to third embodiments, the fourth embodimentexecutes RP conveyance control and following distance control. However,in this embodiment, the RP conveyance control is configured to controlonly the actuation reaction force of the accelerator pedal 71 based onthe risk potential RP and does not control the braking/driving force.

The operation of a vehicle driving assist system 2 in accordance withthe fourth embodiment will now be explained. First, an overview of theoperation will be provided. When following distance control isoperating, the driver's foot is away from the accelerator pedal as shownin diagram (a) of FIG. 19. If the driver depresses the accelerator pedal71 while following distance control is operating, the following distancecontrol is overridden and the vehicle driving assist system 2 switchesto RP conveyance control (diagram (b) of FIG. 19). With a conventionalvehicle driving assist system, the start of RP conveyance control causesthe actuation reaction force of the accelerator pedal to increaserapidly in accordance with the risk potential with respect to thepreceding vehicle. Consequently, the accelerator pedal 71 becomesdifficult to depress and the driver feels as though there is a barrierimpeding the motion of the accelerator pedal 71 (diagram (c) of FIG.19). Afterwards, if the driver still attempts to accelerate, theaccelerator pedal 71 will be stiff due to the actuation reaction forcecorresponding to the risk potential RP and it will be difficult toaccelerate (diagram (d) of FIG. 19).

In short, with a conventional system, when the following distancecontrol is overridden and the system switches to RP conveyance control,the actuation reaction force increases in accordance with the riskpotential RP and the driver cannot easily depress the accelerator pedal71 and accelerate even if such is his or her intent. In other words, itis difficult for the driver to operate the vehicle in accordance withhis or her intent. Conversely, with the fourth embodiment, theaccelerator pedal actuation reaction force is increased graduallystarting from before the following distance control is overridden andoperation of the RP conveyance control starts.

The operation of a vehicle driving assist system 2 in accordance withthe fourth embodiment will now be explained with reference to FIG. 20.FIG. 20 is a flowchart showing a portion of the processing steps of adriving assistance program executed by the controller 200. Morespecifically, the flowchart of FIG. 20 shows the control steps executedwhen the following distance control is overridden and the vehicledriving assist system 2 switches to RP conveyance control. This controlloop is executed continuously once per prescribed time period, e.g.,every 50 msec.

In step S1000, the controller 200 acquires information regarding thevehicle and the surroundings (environment) of the vehicle. Morespecifically, the controller 200 reads in signals from the laser radar10 and the image processing device 40 indicating the headway distance Dand relative velocity Vr between the vehicle and the preceding obstacleand a signal from the vehicle speed sensor 20 indicating the hostvehicle speed V1. In step S1010, the controller 200 reads the detectionsignal from the accelerator pedal stroke sensor 90 and determinesaccelerator pedal actuation state. More specifically, the controller 200acquires the depression amount (actuation amount) Ap of the acceleratorpedal 71 detected by the accelerator pedal stroke sensor 90.Additionally, the depression rate Apv of the accelerator pedal 71 iscalculated by, for example, finding a derivative of the acceleratorpedal actuation amount Ap with respect to time.

In step S1020, the controller 200 acquires an operation signal from thesteering switch unit 100 operated by the driver and determines if boththe RP conveyance control and the following distance control are in theoperating state (ON) based on the acquired operation signal. If the RPconveyance control and the following distance control are both on (ON),the controller 200 proceeds to step S1030. If at least one of thecontrols is off (OFF), the controller 200 ends the control sequence. Ifone or the other of the RP conveyance control and the following distancecontrol is on, the control that is on is executed alone. The details ofthe RP conveyance control and the following distance control are thesame as in the first to third embodiments and explanations thereof areomitted for the sake of brevity.

In step S1030, the controller 200 calculates the risk potential RP. Therisk potential RP with respect to the preceding obstacle can becalculated in the same manner as in the first to third embodiments.However, in this embodiment, the risk potential RP is calculated withEquation 11 shown below using the time to collision TTC and the time toheadway THW between the vehicle and the preceding obstacle.RP=a/THW+b/TTC  (Equation 11)

In Equation 11, the terms “a” and “b” are constants serving toappropriately weight the inverse of the time to headway THW and theinverse of the time to collision TTC. The constants “a” and “b” are setin advance to appropriate values, e.g., a=1 and b=8 (a<b).

In step S1040, the controller 200 calculates an accelerator pedalreaction force command value FA based on the risk potential RPcalculated in step S1030. The accelerator pedal reaction force commandvalue FA is set to be, for example, proportional to the risk potentialRP. It is also possible to calculate the command value FA using the mapshown in FIG. 11 as is done in the first embodiment.

In step S1050, the controller 200 reads the accelerator pedal actuationamount Ap detected by the accelerator pedal stroke sensor 90 anddetermines if the accelerator pedal 71 is being depressed. If theaccelerator pedal 71 is being depressed (i.e., if Ap>0), the controller200 determines that the system 2 is in an override standby state thatoccurs before the following distance control is overridden and proceedsto step S1060. The override standby state is defined to be a state inwhich both the RP conveyance control and the following distance controlare on and the accelerator pedal 71 is being depressed by an amount Apthat is smaller than a prescribed value Apo for determining if theoverride state exists.

In step S1060, the controller 1060 determines if the accelerator pedalreaction force should be modified while the system 2 is in the overridestandby state. An accelerator pedal reaction force modification controlis executed if the following execution conditions are satisfied: (1) thesame preceding vehicle is detected as the preceding obstacle and (2) thevalue of an execution time counter Ct is equal to or smaller than aprescribed amount of time Ct1.

The execution time counter Ct expresses the amount of time over whichthe accelerator pedal reaction force modification control has beenexecuted during the override standby state since the accelerator pedal71 was depressed. The prescribed amount of time (prescribed executiontime counter value) Ct1 is substantially equivalent to the sum of theexecution times of a reaction force modification control A and areaction force modification control B (described later) and is set to amaximum of approximately 30 seconds. If the conditions for executingaccelerator pedal reaction force modification control are satisfied, thecontroller 200 proceeds to step S1070 and increments the execution timecounter Ct.

In step S1080, the controller 200 executes control processing formodifying the accelerator pedal reaction force. The accelerator pedalreaction force modification executed during the override standby stateincludes a reaction force modification control (hereinafter called“reaction force modification control A”) executed after the acceleratorpedal 71 starts being depressed and a reaction force modificationcontrol (hereinafter called “reaction force modification control B”)executed as a transition from the reaction force modification control Ato the normal RP conveyance control based on the risk potential RP. Thecontrol processing executed in order to modify the accelerator pedalreaction force during the override standby state will now be explainedwith reference to the flowchart of FIG. 21.

In step S1181, the controller 200 compares the value of the executiontime counter Ct after incrementing in step S1070 to a prescribed amountof time Ct2. The prescribed amount of time Ct2 expresses the executiontime of the reaction force modification control A and is set inaccordance with the depression rate Apv of the accelerator pedal 71.FIG. 22 shows a graph plotting the prescribed amount time Ct2 versus theaccelerator pedal depression rate Apv. As shown in FIG. 22, the fasterthe accelerator pedal depression rate Apv is, the shorter the prescribedamount of time Ct2 becomes and the shorter the amounted time over whichthe reaction force modification control A is executed. Thus, the morequickly the accelerator pedal 71 is depressed, the shorter the executiontime of the reaction force modification control A becomes and theearlier the shift to the reaction force modification control B occurs.

If the value of execution time counter Ct is smaller than the prescribedamount of time Ct2, then the controller 200 proceeds to step S1183 andcalculates a modified reaction force command value to be used for thereaction force modification control A. For the prescribed amount of timeCt2, the reaction force modification control A functions to limit therisk potential RP-based accelerator pedal actuation reaction force thatwould result with normal RP conveyance control. More specifically, amodified command value Fmodified is calculated with the Equation 12shown below using a modification coefficient Cfm and the reaction forcecommand value FA calculated in step S1040 based on the risk potentialRP.Fmodified=Cfm×FA  (Equation 12)

The modification coefficient Cfm is set to a value equal to or less than1, e.g., 0.5.

FIG. 23 shows plots of the modified command value Fmodified versus thereaction force command value FA. When the reaction force command valueFA is multiplied by a modification coefficient Cfm that is smaller than1, the modified command value Fmodified is smaller than the reactionforce command value FA calculated in accordance with normal RPconveyance control and the slope of the modified command value Fmodifiedwith respect to the reaction force command value FA decreases.

If it determines in step S1181 that the value of the execution timecounter Ct is equal to or larger than the prescribed amount of time Ct2,then the controller 200 proceeds to step S1185 and calculates a modifiedreaction force command value to be used for the reaction forcemodification control B. More specifically, when the prescribed amount oftime Ct2 elapses and the system 2 shifts to the reaction forcemodification control B, the modification coefficient Cfm is returned toa value of 1 in a gradual fashion. For example, the modificationcoefficient Cfm might be gradually increased to a value of 1 (Cfm=1(100%)) at a rate of 5% per second. The reaction force modificationcontrol B calculates the modified command value Fmodified in accordancewith the aforementioned Equation 12 using the gradually increasingmodification coefficient Cfm. The reaction force modification control Bends when the value of the modification coefficient Cfm reaches 1.

After it calculates the modified reaction force command value Fmodifiedin step S1080, the controller 200 proceeds to step S1090. Meanwhile, ifit determines in step S1060 that the conditions for accelerator pedalreaction force modification control are not satisfied, the controller200 proceeds to step S1100. Also, when the reaction force modificationcontrol B ends, the result of step S1060 will be negative and thecontroller 200 will proceed to step S1100 because the execution timecounter Ct will have exceeded the prescribed time Ct1. In step S1100,the controller 200 enters a cancellation phase of the accelerator pedalreaction force modification control. The controller 200 resets theexecution time counter Ct to 0 and sets the modified reaction forcecommand value Fmodified to the accelerator pedal reaction force commandvalue FA calculated in step S1040 as is (i.e., without modification. Inother words, in the cancellation phase, the reaction force control isthe same as during normal RP conveyance control.

In step S1090, the controller 200 sends the modified reaction forcecommand value Fmodified set in step S1080 or S1100 to the acceleratorpedal reaction force control device 70. The accelerator pedal reactionforce control device 70 controls the servomotor 80 based on the commandfrom the controller 200 and thereby controls the actuation reactionforce exerted when the driver operates the accelerator pedal 71. Morespecifically, the accelerator pedal reaction force control device 70causes the accelerator pedal 71 to exert a reaction force equal to thesum of the modified reaction force command value Fmodified and a normalpedal reaction force characteristic that is based on the acceleratorpedal actuation amount Ap.

In step S1110, the controller 200 presents information indicating theoperating states of the RP conveyance control and the following distancecontrol. More specifically, if the reaction force modification control Ais being executed, the controller 200 illuminates the indicator lamp forfollowing distance control and flashes the indicator lamp for RPconveyance control. If the reaction force modification control B isbeing executed, the controller 200 illuminates the indicator lamp forfollowing distance control and flashes the indicator lamp for RPconveyance control slowly. If the reaction force modification control isin the cancellation phase, the controller 200 flashes the indicator lampfor following distance control slowly and illuminates the indicator lampfor RP conveyance control. After the lamps are illuminated, the currentcycle of the control loop ends.

The operational action of the fourth embodiment will now be explainedwith reference to graphs (a) and (b) of FIG. 24. Graph (a) of FIG. 24shows how the modification coefficient Cfm, i.e., the ratio (percentage)of the modified value Fmodified with respect to the reaction forcecommand value FA calculated based on the risk potential RP, varies withtime. Graph (b) of FIG. 24 shows an example of how the reaction forcecommand value FA and the modified value Fmodified.

The time t0 corresponds to the time when accelerator pedal reactionforce modification control A starts after the aforementioned executionconditions are satisfied. During the reaction force modification controlA, the controller 200 outputs a modified command value Fmodified(solid-line curve) that is calculated by limiting the reaction forcecommand value FA (single-dot chain line curve) calculated based on therisk potential RP. At this stage, the modification coefficient Cfm isset to 0.5 such that the modified command value Fmodified is limited to50% of the reaction force command value FA. As a result, the modifiedcommand value Fmodified changes loosely in accordance with changes inthe reaction force command value FA calculated based on the riskpotential RP.

At a time t1, the prescribed amount Ct2 has elapsed since the reactionforce modification control A started and the controller 200 shifts tothe reaction force modification control B. The reaction forcemodification control B causes the modification coefficient Cfm toincrease from 0.5 to 1 at a rate of 5% per second. Thus, the modifiedcommand value Fmodified gradually increases from being limited to 50% ofthe reaction force command value FA (which is based on the riskpotential RP) to being equal to 100% of the reaction force command valueFA. After the modification coefficient Cfm increases to 1, the reactionforce command value FA based on the risk potential RP is used as is andnormal RP conveyance control is executed.

Since the prescribed amount of time Ct2 over which the reaction forcemodification control A is executed is set based on the depression rateApv of the accelerator pedal 71, the control shifts to RP conveyancecontrol quickly when the driver depresses the accelerator pedal 71rapidly in an attempt to accelerator, thus enabling informationregarding the surroundings of the vehicle to be conveyed to the driverthrough the accelerator pedal actuation reaction force.

In addition to the operational effects exhibited by the firstembodiment, the fourth embodiment also achieves the following additionaleffects.

(1) The vehicle driving assist system 2 detects a preparation stateoccurring before the system 2 shifts from a state in which followingdistance control is operating to a state in which RP conveyance controlis operating. If the preparation state is detected, the system 2 limitsthe RP conveyance control. In other words, before shifting fromfollowing distance control to actual RP conveyance control, the system 2executes a limited RP conveyance control. As a result, the fact that thesystem 2 will switch to RP conveyance control can be conveyed to thedriver in advance and the control imposed by the system 2 can beprevented from being at odds with the intent of the driver regardingdriving the vehicle.

(2) The vehicle driving assist system 2 detects the preparation statebased on the actuation state of a driver-operated driving operationdevice. If it detects the preparation state, the system 2 executescontrol that causes the driving operation device to exert a reactionforce that is limited with respect to the actuation reaction force thatwould result under normal RP conveyances control. The driving operationdevice is, for example, an accelerator pedal 71. Thus, morespecifically, when the accelerator pedal 71 is depressed (Ap>0), thesystem 2 detects the preparation state (override standby state) andcauses the accelerator pedal 71 to exert a limited actuation reactionforce. As a result, the accelerator pedal reaction force can beprevented from increasing abruptly at the point in time when the system2 switches to RP conveyance control and the intent of the driver toaccelerate and the control executed by the system 2 can be preventedfrom conflicting with each other.

(3) The vehicle driving assist system 2 continues modifying the reactionforce from the time when the preparation state is detected until aprescribed amount of time Ct1 has elapsed or until the vehicle beginsaccelerating. By limiting the reaction force modification control to theprescribed amount of time Ct1, the reaction force modification controlcan be prevented from diminishing the effect of the RP conveyancecontrol, which is intended to convey the risk potential RP to thedriver. By continuing the reaction force modification control untilacceleration starts, the control executed by the system can be preventedin an effective manner from conflicting with the intent of the driver toaccelerate. The determination as to whether the vehicle is acceleratingcan be accomplished by detecting the acceleration rate of the vehicle.In such a case, an additional condition is added to the aforementionedtwo execution conditions such that when the determination regardingwhether or not to execute reaction force modification control is made instep S1060, the system determines that accelerator pedal reaction forcemodification should be executed during the override standby state if thevehicle is not accelerating.

(4) The controller 200 sets the prescribed amount of time Ct1 over whichit modifies the reaction force during the override standby state in avariable manner based on the accelerator pedal depression rate Apv. Morespecifically, the larger the accelerator pedal depression rate Apv is,the shorter the value to which the prescribed amount of time Ct2 is set.The prescribed amount of time Ct2 is the continuous amount of time overwhich the reaction force modification control A is executed to limit theactuation reaction force. As a result, the faster the driver depressesthe accelerator pedal 71, i.e., the stronger the driver's intent toaccelerate is, the quicker the system switches to RP conveyance control.

(5) The vehicle driving assist system 2 informs the driver that it is inthe override standby state using at least one of the following methods:visual information, sound information, accelerator pedal vibration, andaccelerator pedal clicking. As a result, the driver can be informed thatreaction force modification control is in progress and inappropriateoperation of the vehicle and misunderstandings regarding the operatingstate of the system can be avoided.

First Variation of the Fourth Embodiment

In the fourth embodiment, the limited (modified) command value Fmodifiedis calculated by multiplying the reaction force command value FA by amodification coefficient Cfm. In this variation, during the reactionforce modification control A, the reaction force command value FA islimited with respect to the risk potential RP during the prescribedamount of time Ct2.

FIG. 25 is a graph plotting the modified command value Fmodified versusthe risk potential RP. Normally, the reaction force command value FAincreases as the risk potential RP increases, as indicated with thedotted line in the figure. When the risk potential RP is smaller than aprescribed value RPS, the normal reaction force command value FAcalculated based the risk potential RP is used as the modified commandvalue Fmodified. When the risk potential RP is equal to or larger thanthe prescribed value RPS, the modified command value Fmodified islimited to a value F_RPS that equals the reaction force command value FAcorresponding to the prescribed value RPS.

When the risk potential RP is larger than a prescribed value RPE (>RPS),again, the normal reaction force command value FA calculated based onthe risk potential RP is used as the modified command value Fmodified.The prescribed values RPS and RPE are set to, for example, 1.0 and 3.5,respectively (RPS=1.0 and RPE=3.5).

Thus, in a region of low risk where the risk potential is smaller thanthe prescribed value RPS, the reaction force command value FA based onthe risk potential RP is used as is because it is small enough not tocause the driver to feel that there is something odd about the vehiclebehavior. Conversely, when the risk potential RP is equal to or largerthan the prescribed value RPS, the reaction force command value FA islimited to the prescribed value F_RPS in order to avoid impeding thedriver's ability to operate the pedal. Meanwhile, when the riskpotential RP is equal to or larger than the prescribed value RPE, thedegree of convergence with respect to the preceding obstacle is high andthe reaction force command value FA based on the risk potential RP isused as is in order to quickly urge the driver to perform an appropriatedriving operation.

The reaction force command value FA of the accelerator pedal 71 is setsuch that it increases as the risk potential increases, as indicatedwith the dotted line in FIG. 25. However, when reaction forcemodification control is executed after the override standby state(preparation state) is detected, the actuation reaction force commandvalue FA is fixed at a prescribed value F_RPS and does not change withrespect increases in the risk potential RP. As a result, the actuationreaction force can be limited such that it does not impede the driver'sability to operate the accelerator pedal 71 when the driver attempts todepress the accelerator pedal 71.

Second Variation of the Fourth Embodiment

In this variation, during the reaction force modification control B, therate of change of the modified command value Fmodified is varieddepending on whether the reaction force command value FA is increasingor decreasing. The difference between the reaction force command valueFA calculated in the current cycle and the reaction force command valueFAo calculated in the previous cycle is calculated to obtain a changeamount ΔF(=FA−FAo). A positive change amount ΔF indicates that thereaction force command value FA is increasing and a negative changeamount ΔF indicates the reaction force command value FA is decreasing.

Graphs (a) and (b) of FIG. 26 plot the change amount ΔFmodified of themodified command value Fmodified versus the change amount ΔF of thereaction force command value FA during the reaction force modificationcontrol A and the reaction force modification control B, respectively.The change amount ΔFmodified is a change rate limiter of the modifiedcommand value FAmodified. As shown in graph (a) of FIG. 26, if themodification coefficient Cfm is set to, for example, 0.5 during thereaction force modification control A, then the modified command valuechange rate ΔFmodified will be smaller than when the modificationcoefficient Cfm is set to 1 (indicated with broken line). The changeamount ΔFmodified is set in the same manner both when the reaction forcecommand value FA increases and when the reaction force command value FAdecreases.

When the prescribed time Ct2 has elapsed since the reaction forcemodification control A started, the system 2 shifts to the reactionforce modification control B. As shown in graph (b) of FIG. 26, duringthe reaction force modification control B, a different modified commandvalue change amount ΔFmodified is used depending on whether the reactionforce command value FA is increasing or decreasing. More specifically,the change amount ΔFmodified is calculated using the modificationcoefficient Cfm_increase when the reaction force command value FA isincreasing and the modification Cfm_decrease when the reaction forcecommand value FA is decreasing. The change amount ΔFmodified is set to alarger value when the reaction force command value FA is increasing thanwhen the reaction force command value FA is decreasing.

During the reaction force modification control B, the modified commandvalue Fmodified can be calculated with Equations 13 and 14 shown below.

-   -   When reaction force command value FA is increasing (ΔF>0)        Fmodified=FAo+ΔF×Cfm_increase  (Equation 13)    -   When reaction force command value FA is deceasing (ΔF<0)        Fmodified=FAo+ΔF×Cfm_decrease  (Equation 14)

FIG. 27 shows an example of how the reaction force command value FA andthe modified value Fmodified change over time. During the reaction forcemodification control A, the controller 200 outputs a modified commandvalue Fmodified (solid-line curve) that is calculated by limiting thereaction force command value FA (single-dot chain line curve) calculatedbased on the risk potential RP. The modified command value Fmodifiedchanges loosely in accordance with changes in the reaction force commandvalue FA.

At a time t1, the prescribed amount Ct2 has elapsed since the reactionforce modification control A started and the controller 200 shifts tothe reaction force modification control B. During the modificationcontrol B, the modified command value Fmodified is calculated usingEquation 13 when the reaction force command value FA is increasing andusing Equation 14 when the reaction force command value FA isdecreasing. As a result, the modified command value Fmodified increasesrapidly with respect to increases in the reaction force command value FAand decreases gradually with respect to decreases in the reaction forcecommand value FA. The reaction force modification control B ends whenthe modified command value Fmodified has increased to where it equalsthe reaction force command value FA.

Thus the controller 200 changes the manner in which it modifies thereaction force after the override standby state is detected depending onwhether the actuation reaction force is increasing or decreasing. As aresult, the control can be executed in a smooth manner without causingthe driver to experience a feeling that something is odd about thevehicle performance when the control returns (reaction forcemodification control B) to normal RP conveyance control from a state inwhich the actuation reaction force is limited (reaction forcemodification control A).

Third Variation of the Fourth Embodiment

In first variation of the fourth embodiment, during the reaction forcemodification control A, the reaction force command value FA is limitedto a value F_RPS, which is the reaction force command value FAcorresponding to a prescribed risk potential value RPS when the riskpotential RP is equal to or larger than the prescribed value RPS andsmaller than a prescribed value RPE. In this variation, even during thereaction force modification control B the reaction force command valueFA is limited to the value F_RPS corresponding to the prescribed valueRPS when the risk potential RP satisfies the condition RPS≦RP<RPE.However, during the reaction force modification control B, theprescribed value F_RPS corresponding to the prescribed value RPS isgradually increased.

FIG. 28 shows an example of how the reaction force command value FAcalculated based on the risk potential RP and the limited (modified)command value Fmodified change over time. During the reaction forcemodification control A, the modified reaction force command valueFmodified is limited to a value F_RPS corresponding to a fixedprescribed value RPS (as indicated with the solid-line curve) when thecondition RPS≦RP<RPE is satisfied. Once it shifts to the reaction forcemodification control B, the controller 200 gradually increases theprescribed value RPS at a rate of, for example, 0.25 per second. As aresult, the modified reaction force command) value Fmodified (=F_RPS)corresponding to the prescribed value RPS gradually increases. Thereaction force modification control B ends when the prescribed value RPShas increased to the prescribed value RPE. The change amount of theprescribed value RPS is set, for example, such that the prescribed valueRPS reaches the prescribed value RPE in approximately 10 seconds.

Fifth Embodiment

A vehicle driving assist system in accordance with a fifth embodiment ofthe present invention will now be explained. FIG. 29 is a system diagramof a vehicle driving assist system 3 in accordance with the fifthembodiment. In FIG. 29, parts having the same functions as the parts ofthe fourth embodiment shown in FIG. 18 are indicated with the samereference numerals. The fifth embodiment will be explained chiefly bydescribing its differences with respect to the fourth embodiment.

As shown in FIG. 29, the vehicle driving assist system 3 in accordancewith the fifth embodiment is further equipped with an alarm soundissuing device 530 and a pedal vibrator 580 (i.e., a transducer or othervibration generating device). The alarm sound issuing device 530 is, forexample, an alarm sound device configured to generate an alarm sound inresponse to a command from the information presentation controller 510.The pedal vibrator 580 is, for example, mounted to the pedal surface ofthe accelerator pedal 71 and configured to vibrate the accelerator pedal71 in response to a command from the information presentation controller510.

Also, the RP conveyance control of a vehicle driving assistance device 3in accordance with the fifth embodiment is contrived to impose adeceleration on the vehicle in addition to controlling the actuationreaction force in accordance with the risk potential RP. When adeceleration is generated against the vehicle as part of the RPconveyance control, a target deceleration rate is calculated based onthe repulsive force of an imaginary elastic body provided on the frontof the vehicle, similarly to the first embodiment.

In the fifth embodiment, the manner in which the reaction force ismodified is changed depending on the state of the vehicle driving assistsystem. More specifically, the content of the reaction force control ischanged at the point in time of a transition between following distancecontrol and RP conveyance control and at a point in time when thevehicle starts accelerating.

The operation of a vehicle driving assist system 3 in accordance withthe fifth embodiment will now be explained with reference to FIG. 30.FIG. 30 is a flowchart showing a portion of the processing steps of adriving assistance program executed by the controller 210. Morespecifically, the flowchart of FIG. 30 shows the control steps executedwhen the following distance control is overridden and the vehicledriving assist system 3 switches to RP conveyance control. This controlloop is executed continuously once per prescribed time period, e.g.,every 50 msec. The processing of the steps S2000 to S2070 is the same asin the steps S1000 to S1070 of the flowchart shown in FIG. 20 andexplanations of these steps are omitted for the sake of brevity.

In step S2080, the controller 210 determines if the actuation amount Apof the accelerator pedal operated by the driver is larger than aprescribed value Apo. The prescribed value Apo is a threshold value fordetermining if the system will shift from a state in which the followingdistance control is operating to a state in which the following distancecontrol has been overridden due to depression of the accelerator pedal71. The prescribed value Apo is set to a stroke amount of theaccelerator pedal 71 required to achieve a target vehicle speed duringfollowing distance control.

If the actuation amount Ap is equal to or smaller than the prescribedvalue Apo (Ap≦Apo), then the controller 210 determines that theaccelerator pedal 71 is being depressed but the following distancecontrol is not overridden, i.e., that the system 3 is in the overridestandby state, and proceeds to step S2120. If the actuation amount Ap islarger than the prescribed value Apo (Ap>Apo), then the controller 210determines that the following distance control has been overridden andproceeds to step S2090.

In step S2090, the controller 210 compares the driver's requesteddriving force drv_trq corresponding to the accelerator pedal actuationamount Ap to a repulsive torque Repulsive_trq. The repulsive torqueRepulsive_trq is calculated as a repulsive force of an imaginary elasticbody 170 provided on the front of the vehicle using the map shown inFIG. 12, similarly to the first to third embodiments. Thus, therepulsive torque rf1 calculated based on the map of FIG. 12 is used asthe repulsive torque Repulsive_trq. If the conditiondrv_trq≦Repulsive_trq exists, then the repulsive torque Repulsive_trqset by the RP conveyance control is larger than the driver's requesteddriving force drv_trq and the host vehicle does not accelerate eventhough the accelerator pedal 71 is being depressed. Thus, the controller210 determines that the system 3 is in an RP conveyance decelerationstate in which the vehicle decelerates during the RP conveyance control.The controller 210 then proceeds to step 2130.

Meanwhile, if the condition drv_trq>Repulsive_trq exists, then thedriver's requested driving force drv_trq is larger than the repulsivetorque Repulsive_trq set by the RP conveyance control. Therefore, thecontroller 210 determines that the system 3 is in an RP conveyanceacceleration state in which the vehicle accelerates during RP conveyancecontrol. The controller 210 then proceeds to step 2100.

In step S2120, the controller 210 sets the control content of a control1 to be executed during the override standby state. The control 1 servesto make the driver aware that the following distance control isoperating while making it easy for overriding to occur. The controlprocessing executed in order to set the control content of the control 1will now be explained with reference to the flowchart of FIG. 31. Instep S2112, the controller 210 determines if the value of the executiontime counter Ct is smaller than a prescribed value Ct2. If so (Yes), thecontroller 210 proceeds to step S2114 and calculates the modifiedcommand value Fmodified in accordance with the reaction forcemodification control A. The modified command value Fmodified iscalculated using the aforementioned Equation 12. The modificationcoefficient Cfm of Equation 12 is set to, for example, 0.5.

If in step S2112 the controller 210 determines that the value of theexecution time counter Ct is equal to or larger than the prescribedvalue Ct2, the controller 210 proceeds to step S2116 and calculates themodified command value Fmodified in accordance with the reaction forcemodification control B. The modified command value Fmodified iscalculated using the aforementioned Equations 13 and 14. Themodification coefficient Cfm_increase for when the reaction forcecommand value FA is increasing is set to, for example, 0.6 and themodification coefficient Cfm_decrease for when the reaction forcecommand value FA is decreasing is set to, for example, 0.4.

Additionally, during the control 1, the controller 210 informs thedriver that reaction force modification control is being executed duringfollowing distance control by generating a click in the acceleratorpedal 71 and issuing visual information. If the driver's foot has juststarted to depress the accelerator pedal 71 from a state in which it wasreleased from the accelerator pedal 71 during following distancecontrol, the controller 210 generates a single pulse-like click in theaccelerator pedal 71. The magnitude and duration of the click reactionforce (supplemental reaction force) are set in advance to appropriatevalues that enable the driver to recognize that the reaction force ofthe accelerator pedal 71 changed. The controller 210 also flashes theindicator lamp for following distance control slowly and flashes theindicator lamp for RP conveyance control.

In step S2130, the controller 210 sets the control content of a control2 to executed during the RP conveyance deceleration state. The control 2serves to make the driver aware that the RP conveyance control isoperating and the vehicle is not accelerating while making it easy foroverriding to occur. The control processing executed in order to set thecontrol content of the control 2 will now be explained with reference tothe flowchart of FIG. 32. In step S2122, the controller 210 determinesif the value of the execution time counter Ct is smaller than aprescribed value Ct2. If so (Yes), the controller 210 proceeds to stepS2124 and calculates the modified command value Fmodified in accordancewith the reaction force modification control A. The modified commandvalue Fmodified is calculated using the aforementioned Equation 12. Themodification coefficient Cfm of Equation 12 is set to, for example, 0.5.

If in step S2122 the controller 210 determines that the value of theexecution time counter Ct is equal to or larger than the prescribedvalue Ct2, the controller 210 proceeds to step S2126 and calculates themodified command value Fmodified in accordance with the reaction forcemodification control B. The modified command value Fmodified iscalculated using the aforementioned Equations 13 and 14. Themodification coefficient Cfm_increase for when the reaction forcecommand value FA is increasing is set to, for example, 0.7 and themodification coefficient Cfm_decrease for when the reaction forcecommand value FA is decreasing is set to, for example, 0.4.

Additionally, during the control 2, the system 3 informs the driver thatreaction force modification control is being executed while the vehicleis decelerating during RP conveyance control by issuing visual and soundinformation and generating a vibration in the accelerator pedal 71. Morespecifically, the indicator lamp for following distance control and theindicator lamp for RP conveyance control are flashed, and an alarm sound(e.g., a “beep” sound) is emitted from the alarm sound deviceimmediately after the controller 210 switches to the control 2. Theaccelerator pedal 71 is also vibrated. The period and amplitude of thevibration are set in advance to appropriate values that enable thedriver to recognize that the accelerator pedal 71 is vibrating.

In step S2100, the controller 210 sets the control content of a control3 to be executed during the RP conveyance acceleration state. Thecontrol 3 serves to make the driver aware that the RP conveyance controlis operating and reaction force modification control is being executedwhile making it easy for overriding to occur. The control processingexecuted in order to set the control content of the control 3 will nowbe explained with reference to the flowchart of FIG. 33. In step S2132,the controller 210 determines if the value of the execution time counterCt is smaller than a prescribed value Ct2. If so (Yes), the controller210 proceeds to step S2134 and calculates the modified command valueFmodified in accordance with the reaction force modification control A.The modified command value Fmodified is calculated using theaforementioned Equation 12. The modification coefficient Cfm of Equation12 is set to, for example, 0.5.

If in step S2132 the controller 210 determines that the value of theexecution time counter Ct is equal to or larger than the prescribedvalue Ct2, the controller 210 proceeds to step S2136 and calculates themodified command value Fmodified in accordance with the reaction forcemodification control B. The modified command value Fmodified iscalculated using the aforementioned Equations 13 and 14. Themodification coefficient Cfm_increase for when the reaction forcecommand value FA is increasing is set to, for example, 0.7 and themodification coefficient Cfm_decrease for when the reaction forcecommand value FA is decreasing is set to, for example, 0.5.

Additionally, during the control 3, the controller 210 informs thedriver that reaction force modification control is being executed whilethe vehicle is accelerating during RP conveyance control by issuingvisual and sound information and generating a vibration in theaccelerator pedal 71. More specifically, the indicator lamp forfollowing distance control is turned off, the indicator lamp for RPconveyance control is flashed, and an alarm sound (e.g., a “beep” sound)is emitted from the alarm sound device immediately after the controller210 switches to the control 3. The accelerator pedal 71 is alsovibrated. The period and amplitude of the vibration are set in advanceto appropriate values that enable the driver to recognize that theaccelerator pedal 71 is vibrating.

After the content of the reaction force modification control isdetermined based on the state of the system, the controller 210 proceedsto step S2140. If in step S2060 it determines that the executionconditions for reaction force modification control have not beensatisfied, then the controller 210 proceeds to step S2110 and enters thecancellation phase of the acceleration pedal reaction force modificationcontrol.

In step S2140, the controller 210 sends the modified reaction forcecommand value Fmodified set by the control 1 of step S2120, the control2 of step S2130, the control 3 of the step S2100, or the cancellationphase of step S1100 to the accelerator pedal reaction force controldevice 70.

In step S2150, the controller 210 sends commands to the informationpresentation controller 510 in accordance with the control content ofthe control 1, control 2, control 3, or cancellation phase. Theinformation presentation controller 510 controls the informationpresenting device 520, the alarm sound issuing device 530, and the pedalvibrator 580 based on the commands from the controller 210 and, thereby,causes visual information, sound information, and a pedal vibration orclick to be issued in accordance with the set control content.

In step S2160, the controller 210 calculates a target deceleration rate(deceleration rate command value) to be used during RP conveyancecontrol. The target deceleration rate is calculated based on therepulsive force of an imaginary elastic body 170 like that shown in thediagrams (a) and (b) of FIG. 9, similarly to the first embodiment. Instep S2170, the controller 210 sends the deceleration rate command valuecalculated in step S2160 to the engine controller 50 and the brakecontroller 60. After the command values are sent, the current cycle ofthe control loop ends.

Variations 1 to 3 of the fourth embodiment can also be employed in thecontrols 1 to 3 and the same effects can be obtained when thesevariations are used.

The fifth embodiment just described can provide the followingoperational effects in addition to the effects provided by the first tofourth embodiments.

(1) The controller 210 changes the manner in which it modifies thereaction force after the override standby state is detected depending onthe control states of the RP conveyance control and the followingdistance control. As a result, the driver can be informed as to whatkind of control the vehicle driving assist system 3 is executing and thereaction force can be modified in a manner that is well-matched to thecontrol that is being executed.

(2) The controller 210 executes the reaction force modificationdifferently depending on whether the system 3 is in a state in whichfollowing distance control is operating, a state in which RP conveyancecontrol is operating and the vehicle is not accelerating, or a state inwhich RP conveyance control is operating and the vehicle isaccelerating. As a result, the reaction force modification control canbe executed in a manner that is well-matched to changes in the state ofthe system and vehicle, such as changing to or from a state in whichfollowing distance control is operating or a state in which the vehicleis accelerating.

Sixth Embodiment

A vehicle driving assist system in accordance with a sixth embodiment ofthe present invention will now be explained. The basic constituentfeatures of a vehicle driving assist system in accordance with the sixthembodiment are the same as those of the fourth embodiment shown in FIG.18. The fifth embodiment will be explained chiefly by describing itsdifferences with respect to the fourth embodiment.

In the sixth embodiment, the content of the reaction force modificationcontrol is varied depending on the relative positioning and movement ofthe vehicle with respect to an obstacle, e.g., a preceding vehicle,existing in front of the vehicle. More specifically, the vehicle drivingassist system determines what kind of following situation exists whenthe driver depresses the accelerator pedal 71 and overrides thefollowing distance control and to set the control content in accordancewith the determined following situation.

The operation of a vehicle driving assist system 3 in accordance withthe sixth embodiment will now be explained with reference to FIG. 34.FIG. 34 is a flowchart showing a portion of the processing steps of adriving assistance program executed by the controller 200. Morespecifically, the flowchart of FIG. 34 shows the control steps executedwhen the following distance control is overridden and the vehicledriving assist system 3 switches to RP conveyance control. This controlloop is executed continuously once per prescribed time period, e.g.,every 50 msec. The processing of the steps S3000 to S3070 is the same asin the steps S1000 to S1070 of the flowchart shown in FIG. 20 andexplanations of these steps are omitted for the sake of brevity.

In step S3080, the controller 200 determines the following situationexisting when the accelerator pedal 71 was depressed and the followingdistance control was overridden. The determination is made by comparingthe set time to headway THWacc that was set by the following distancecontrol to the current time to headway THW between the vehicle and thepreceding vehicle. The controller 200 sets the set time to headwayTHWacc as a fixed value and updates the current time to headway THWconsecutively while determining if the relationship expressed byEquation 15 below is satisfied.THWacc×0.9<THW<THWacc×1.1  (Equation 15)

If the relationship expressed in Equation 15 is satisfied continuouslyfor a prescribed amount of time (e.g., 5 seconds) or if a directional(turn signal) operation or other operation indicating that the driverintends to change lanes is detected, then the controller 200 determinesthat the driver is depressing the accelerator pedal 71 in an attempt topass the preceding vehicle. Otherwise, the controller 200 determinesthat the driver is depressing the accelerator pedal 71 in an attempt toapproach nearer to the preceding vehicle.

If it determines that the following situation is a pass situation, thenthe controller 200 proceeds to step S3090 and executes an acceleratorpedal reaction force modification control tailored to pass situations.The reaction force modification control for pass situations will now beexplained with reference to the flowchart of FIG. 35. In step S3091, thecontroller 200 determines if the value of the execution time counter Ctis smaller than a prescribed value Ct2. If so (Yes), then the controller210 proceeds to step S3093 and calculates the modified command valueFmodified in accordance with the reaction force modification control A.

The modified command value Fmodified is calculated in accordance withthe previously described FIG. 25. When the condition RP<RPS exists, thenormal reaction force command value FA calculated based on the riskpotential RP is used as the modified command value Fmodified. When thecondition RP≧RPS exists, the modified command value Fmodified is limitedto a value F_RPS that equals the reaction force command value FAcorresponding to the prescribed value RPS. When the conditions RP≧RPE(>RPS) exists, again, the normal reaction force command value FAcalculated based on the risk potential RP is used as the modifiedcommand value Fmodified.

If in step S3091 the controller 200 determines that the value of theexecution time counter Ct is equal to or larger than the prescribedvalue Ct2, then the controller 200 proceeds to step S3095 and calculatesthe modified command value Fmodified in accordance with the reactionforce modification control B. Similarly to the reaction forcemodification control A, during the reaction force modification control Bthe reaction force command value FA is limited to the value F_RPScorresponding to the prescribed value RPS when the risk potential RPsatisfies the condition RPS≦RP<RPE. However, during the reaction forcemodification control B, the prescribed value F_RPS corresponding to theprescribed value RPS is gradually increased. More specifically, after itshifts to the reaction force modification control B, the controller 200gradually increases the prescribed value RPS at a rate of, for example,0.25 per second and, thereby, increases the value F_RPS of the reactionforce command value FA corresponding to the prescribed value RPS (FIG.28). The reaction force modification control B ends when the prescribedvalue RPS has increased to the prescribed value RPE.

If it determines that the following situation is an approach situation,then the controller 200 proceeds to step S3120 and executes anaccelerator pedal reaction force modification control tailored toapproach situations. The reaction force modification control forapproach situations will now be explained with reference to theflowchart of FIG. 36. In step S3121, the controller 200 determines ifthe value of the execution time counter Ct is smaller than a prescribedvalue Ct2. If so (Yes), the controller 200 proceeds to step S3123 andcalculates the modified command value Fmodified in accordance with thereaction force modification control A. The modified command valueFmodified is calculated using the aforementioned Equation 12. Themodification coefficient Cfm of Equation 12 is set to, for example, 0.5.

If in step S3121 the controller 200 determines that the value of theexecution time counter Ct is equal to or larger than the prescribedvalue Ct2, then the controller 200 proceeds to step S3125 and calculatesthe modified command value Fmodified in accordance with the reactionforce modification control B. After it shifts to the reaction forcemodification control B, the controller 200 gradually increases themodification coefficient Cfm to 1. For example, the modificationcoefficient Cfm might be gradually increased to a value of 1 (Cfm=1(100%)) at a rate of 5% per second while calculating the modifiedcommand value Fmodified using Equation 12. The reaction forcemodification control B ends when the value of the modificationcoefficient Cfm reaches 1.

After the content of the reaction force modification control isdetermined based on the following situation with respect to thepreceding vehicle, the controller 200 proceeds to step S3100. If itdetermines in step S3060 that the execution conditions for reactionforce modification control have not been satisfied, then the controller200 proceeds to step S3110 and enters the cancellation phase of theacceleration pedal reaction force modification control.

In step S3100, the controller 200 sends the modified reaction forcecommand value Fmodified set in step S3090, S3120, or S3110 to theaccelerator pedal reaction force control device 70.

In step S3130, the controller 200 presents information indicating theoperating states of the RP conveyance control and the following distancecontrol. More specifically, if the reaction force modification control Ais being executed, the controller 200 illuminates the indicator lamp forfollowing distance control and flashes the indicator lamp for RPconveyance control. If the reaction force modification control B isbeing executed, then the controller 200 illuminates the indicator lampfor following distance control and flashes the indicator lamp for RPconveyance control slowly. If the reaction force modification control isin the cancellation phase, then the controller 200 flashes the indicatorlamp for following distance control slowly and illuminates the indicatorlamp for RP conveyance control. After the command values are sent, thecurrent cycle of the control loop ends.

The method of determining if the driver intends to change lanes is notlimited to detecting a change in the time to headway THW or detectingthe operation of a directional. Various other methods can also be used.For example, the system can be configured to estimate the driver'sintent regarding changing lanes based on the degree of agreement betweenan actual actuation amount of a driving operation device operated by thedriver and a plurality of actuation amounts of the driving operationdevice operated by imaginary drivers.

The sixth embodiment just described can provide the followingoperational effects in addition to the effects provided by the first tofifth embodiments.

(1) The controller 200 changes the manner in which it modifies thereaction force after the override standby state is detected depending onthe traveling situation of the vehicle. As a result, the reaction forcemodification can be tailored to the traveling situation of the vehicle.

(2) The traveling situations include a pass situation in which thevehicle passes an obstacle and an approach situation in which thevehicle draws closer to the obstacle. As a result, an appropriatereaction force modification can be executed in accordance with whetherthe driver is performing a driving operation in order to pass theobstacle or a driving operation in order to approach the obstacle.

Seventh Embodiment

A vehicle driving assist system in accordance with a seventh embodimentof the present invention will now be explained. The basic constituentfeatures of a vehicle driving assist system in accordance with theseventh embodiment are the same as those of the fifth embodiment shownin FIG. 29. The seventh embodiment will be explained chiefly bydescribing its differences with respect to the fifth embodiment. In theseventh embodiment, similarly to the first embodiment, when thefollowing distance control is overridden and the system switches to RPconveyance control, the target deceleration rate is adjusted to preventthe driver from experiencing a feeling that something is odd about thevehicle behavior due to the deceleration rate becoming larger than itwas during the following distance control.

In the seventh embodiment, the system distinguishes among threedifferent states that can occur when the following distance control isoverridden due to the accelerator pedal 71 being depressed. Morespecifically, similarly to the fifth embodiment, the systemdistinguishes among an override standby state (accelerator pedalactuation amount Ap≦Apo) in which the accelerator pedal 71 is beingdepressed but the following distance control is not overridden, an RPconveyance deceleration state (drv_trq≦Repulsive_trq) in which therepulsive torque Repulsive_trq set by the RP conveyance control islarger than the driver's requested driving force drv_trq, and an RPconveyance acceleration state (drv_trq>Repulsive_trq) in which thedriver's requested driving force drv_trq is larger than the repulsivetorque Repulsive_trq set by the RP conveyance control.

However, adjusting the target deceleration rate when the followingdistance control is overridden makes the RP conveyance decelerationstate difficult to recognize based on the deceleration of the vehicleand makes it difficult for the driver to know that the system hasswitched from the override standby state of the following distancecontrol to the RP conveyance control. Therefore, the seventh embodimentchanges the content of the reaction force modification control inaccordance with the aforementioned states such that the driver caneasily recognize the operating state of the system.

The operation of a vehicle driving assist system 3 in accordance withthe seventh embodiment will now be explained with reference to FIG. 37.FIG. 37 is a flowchart showing a portion of the processing steps of adriving assistance program executed by the controller 210. Morespecifically, the flowchart of FIG. 37 shows the control steps executedwhen the following distance control is overridden and the vehicledriving assist system 3 switches to RP conveyance control. This controlloop is executed continuously once per prescribed time period, e.g.,every 50 msec. The processing of the steps S4000 to S4070 is the same asin the steps S1000 to S1070 of the flowchart shown in FIG. 20 andexplanations of these steps are omitted for the sake of brevity.

In step S4080, the controller 210 compares the accelerator pedalactuation amount Ap to a prescribed value Apo. If the actuation amountAp is equal to or smaller than the prescribed value Apo (Ap≦Apo), thecontroller 210 determines that the override standby state exists andproceeds to step S4120. If the actuation amount Ap is larger than theprescribed value Apo (Ap>Apo), then the controller 210 determines thatthe following distance control has been overridden and proceeds to stepS4090. In step S4090, the controller 210 compares the driver's requesteddriving force drv_trq corresponding to the accelerator pedal actuationamount SA to a repulsive torque Repulsive_trq. If the conditiondrv_trq≦Repulsive_trq exists, then the controller 210 determines thatthe RP conveyance deceleration state exists and proceeds to step S4130.If the condition drv_trq>Repulsive_trq exists, then the controller 210determines that the RP conveyance acceleration state exists and proceedsto step S4100.

In step S4120, the controller 210 sets the control content for a control1′ to be executed during the override standby state. In the control 1′,the actuation reaction force dictated by the RP conveyance control isnot generated so that the driver can easily recognize that the followingdistance control is operating. More specifically, the modified commandvalue Fmodified is set to 0 (Fmodified=0) regardless of the value of theexecution time counter Ct.

If the driver's foot just started to depress the accelerator pedal 71from a state in which it was released from the accelerator pedal 71during following distance control, the system 3 generates a singlepulse-like click in the accelerator pedal 71. The magnitude and durationof the click reaction force (supplemental reaction force) are set inadvance to appropriate values that enable the driver to recognize thatthe reaction force of the accelerator pedal 71 changed. The controller210 also flashes the indicator lamp for following distance controlslowly and flashes the indicator lamp for RP conveyance control.

In step S4130, the controller 210 sets the control content of a control2′ to be executed during the RP conveyance deceleration state. When thesystem changes from the override standby state to the RP conveyancedeceleration state, the controller 210 starts executing reaction forcemodification control. The processing executed in order to set thecontrol content of the control 2′ will now be explained with referenceto the flowchart of FIG. 38. In step S4122, then the controller 210determines if the value of the execution time counter Ct is smaller thana prescribed value Ct2. If so (Yes), then the controller 210 proceeds tostep S4124 and calculates the modified command value Fmodified inaccordance with the reaction force modification control A. The modifiedcommand value Fmodified is calculated using the aforementioned Equation12. The modification coefficient Cfm of Equation 12 is set to, forexample, 0.5.

If in step S4122 the controller 210 determines that the value of theexecution time counter Ct is equal to or larger than the prescribedvalue Ct2, the controller 210 proceeds to step S4126 and calculates themodified command value Fmodified in accordance with the reaction forcemodification control B. The modified command value Fmodified iscalculated using the aforementioned Equations 13 and 14. Themodification coefficient Cfm_increase for when the reaction forcecommand value FA is increasing is set to, for example, 0.7 and themodification coefficient Cfm_decrease for when the reaction forcecommand value FA is decreasing is set to, for example, 0.4.

Additionally, the indicator lamp for following distance control and theindicator lamp for RP conveyance control are flashed, and an alarm sound(e.g., a “beep” sound) is emitted from the alarm sound deviceimmediately after the controller 210 switches to the control 2′. Theaccelerator pedal 71 is also vibrated. The period and amplitude of thevibration are set in advance to appropriate values that enable thedriver to recognize that the accelerator pedal 71 is vibrating.

In step S4100, the controller 210 sets the control content of a control3′ to be executed during the RP conveyance acceleration state. Theprocessing executed in order to set the control content of the control3′ will now be explained with reference to the flowchart of FIG. 39. Instep S4132, the controller 210 determines if the value of the executiontime counter Ct is smaller than a prescribed value Ct2. If so (Yes), thecontroller 210 proceeds to step S4134 and calculates the modifiedcommand value Fmodified in accordance with the reaction forcemodification control A. The modified command value Fmodified iscalculated using the aforementioned Equation 12. The modificationcoefficient Cfm of Equation 12 is set to, for example, 0.5.

If in step S4132 the controller 210 determines that the value of theexecution time counter Ct is equal to or larger than the prescribedvalue Ct2, then the controller 210 proceeds to step S4136 and calculatesthe modified command value Fmodified in accordance with the reactionforce modification control B. The modified command value Fmodified iscalculated using the aforementioned Equations 13 and 14. Themodification coefficient Cfm_increase for when the reaction forcecommand value FA is increasing is set to, for example, 0.7 and themodification coefficient Cfm_decrease for when the reaction forcecommand value FA is decreasing is set to, for example, 0.5.

Additionally, the indicator lamp for following distance control isturned off, the indicator lamp for RP conveyance control is flashed, andthe accelerator pedal 71 is vibrated. The period and amplitude of thevibration are set in advance to appropriate values that enable thedriver to recognize that the accelerator pedal 71 is vibrating.

After the content of the reaction force modification control isdetermined based on the state of the system, the controller 210 proceedsto step S4140. If it determines in step S4060 that the executionconditions for reaction force modification control have not beensatisfied, then the controller 210 proceeds to step S4110 and enters thecancellation phase of the acceleration pedal reaction force modificationcontrol.

In step S4140, the controller 210 sends the modified reaction forcecommand value Fmodified set by the control 1′ of step S4120, the control2′ of step S4130, the control 3′ of the step S4100, or the cancellationphase of step S4100 to the accelerator pedal reaction force controldevice 70.

In step S4150, the controller 210 sends commands to the informationpresentation controller 510 in accordance with the control content ofthe control 1′, control 2′, control 3′, or cancellation phase. Theinformation presentation controller 510 controls the informationpresenting device 520, the alarm sound issuing device 530, and the pedalvibrator 580 based on the commands from the controller 210 and, thereby,causes visual information, sound information, and a pedal vibration orclick to be issued in accordance with the set control content.

In step S4160, the controller 210 calculates a target deceleration rate(deceleration rate command value) to be used during RP conveyancecontrol. Similarly to the first embodiment, the method of calculatingthe target deceleration rate is adjusted such that the driver does notdoes not experience a feeling that something is odd about the vehiclebehavior when the following distance control is overridden. In stepS4170, the controller 210 sends the deceleration rate command valuecalculated in step S4160 to the engine controller 50 and the brakecontroller 60. After the command values are sent, the current cycle ofthe control loop ends.

The seventh embodiment just described can provide the followingoperational effects in addition to the effects provided by the first tosixth embodiments.

When it detects a transition from a state in which the followingdistance control was operating to a state in which the RP conveyancecontrol is operating, the controller 210 modifies the braking/drivingforce controlled by the RP conveyance control in addition to modifyingthe actuation reaction force. As a result, the system can shift smoothlyto RP conveyance control even when the RP conveyance control iscontrived to convey the risk potential RP via both the actuationreaction force and the braking/driving force.

Eighth Embodiment

A vehicle driving assist system in accordance with an eighth embodimentof the present invention will now be explained. The basic constituentfeatures of a vehicle driving assist system in accordance with theeighth embodiment are the same as those of the fourth embodiment shownin FIG. 18. The eighth embodiment will be explained chiefly bydescribing its differences with respect to the fourth embodiment.

In the previously described fourth embodiment, when the driver depressesthe accelerator pedal 71 during following distance control, a reactionforce modification control is executed so as to generate an actuationreaction force in the accelerator pedal 71 before the system switches toRP conveyance control. During the reaction force modification control, amodified reaction force command value Fmodified is calculated bylimiting the reaction force command value FA calculated based on therisk potential RP. In the eighth embodiment, substantially the sameeffect is obtained by modifying the risk potential RP.

The operation of a vehicle driving assist system 2 in accordance withthe eighth embodiment will now be explained with reference to FIG. 40.FIG. 40 is a flowchart showing a portion of the processing steps of adriving assistance program executed by the controller 200. Morespecifically, the flowchart of FIG. 37 shows the control steps executedwhen the following distance control is overridden and the vehicledriving assist system 3 switches to RP conveyance control. This controlloop is executed continuously once per prescribed time period, e.g.,every 50 msec. The processing of the steps S5000 to S5050 is the same asin the steps S1000 to S1050 of the flowchart shown in FIG. 20 andexplanations of these steps are omitted for the sake of brevity.However, a calculation of the accelerator pedal actuation reaction forcecommand value FA corresponding to step S1040 is not included in thisembodiment.

In step S5060, the controller 200 determines if it should modify therisk potential RP. A control for modifying the risk potential RP isexecuted if the following execution conditions are satisfied: (1) thesame preceding vehicle is detected as the preceding obstacle and (2) thevalue of an execution time counter Ct is equal to or smaller than aprescribed amount of time Ct1. If the conditions for executingaccelerator pedal reaction force modification control are satisfied,then the controller 200 proceeds to step S5070 and increments theexecution time counter Ct.

In step S5080, the controller 200 executes control processing formodifying the risk potential. The risk potential modification executedincludes a risk potential modification control (hereinafter called “riskpotential modification control A”) executed after the accelerator pedal71 starts being depressed and a risk potential modification control(hereinafter called “risk potential modification control B”) executed asa transition from the risk potential modification control A to thenormal RP conveyance control based on the risk potential RP. The controlprocessing executed in order to modify the risk potential RP will now beexplained with reference to the flowchart of FIG. 41.

In step S5181, the controller 200 compares the value of the executiontime counter Ct to a prescribed amount of time Ct2. If the value of theexecution time counter Ct is smaller than the prescribed amount of timeCt2, then the controller 200 proceeds to step S5183 and calculates amodified risk potential value RPmodified to be used for the riskpotential modification control A. The risk potential modificationcontrol A serves to limit the risk potential RP calculated based on therunning condition and traveling environment of the vehicle for theduration of the prescribed amount of time Ct2. For example, as shown inEquation 16 below, the modified value RPmodified can be calculated asone half the risk potential RP calculated in step S5030.RPmodified=½×RP=½{A/THW+B/TTC}  (Equation 16)

If it determines in step S5181 that the value of the execution timecounter Ct is equal to or larger than the prescribed amount of time Ct2,then the controller 200 proceeds to step S5185 and calculates a modifiedrisk potential value RPmodified to be used for the risk potentialmodification control B. More specifically, when the prescribed amount oftime Ct2 elapses and the system 2 shifts to the risk potentialmodification control B, the modified risk potential value RPmodified isgradually returned from one half of the risk potential RP to the fullrisk potential RP. The risk potential modification control B ends whenthe modified value RPmodified equals the risk potential RP(RPmodified=RP).

After the controller 200 calculates the modified risk potential RP instep S5080, the controller 200 proceeds to step S5085. Meanwhile, if itdetermines in step S5060 that the execution conditions for riskpotential modification control are not satisfied, then the controller200 proceeds to step S5100. Also, when the risk potential modificationcontrol B ends, the result of step S5060 will be negative and thecontroller 200 will proceed to step S5100 because the execution timecounter Ct will have exceeded the prescribed time Ct1. In step S5100,the controller 200 shifts to a cancellation phase of the risk potentialmodification control. The controller 200 resets the execution timecounter Ct to 0 and sets the modified reaction force command valueRPmodified to the risk potential RP calculated in step S5030 as is(i.e., without modification).

In step S5085, the controller 200 uses the risk potential modificationvalue RPmodified calculated in step S5080 or S5100 to calculate theaccelerator pedal reaction force command value FA. Similarly to thefourth embodiment, the accelerator pedal reaction force command value FAis set to be, for example, proportional to the modified risk potentialvalue RPmodified.

In step S5090, the controller 200 sends the reaction force controlcommand value FA calculated in step S5085 to the accelerator pedalreaction force control device 70. In step S5110, the controller 200presents information indicating the operating states of the RPconveyance control and the following distance control. Morespecifically, for example, if the risk potential modification control Ais being executed, the controller 200 illuminates the indicator lamp forfollowing distance control and flashes the indicator lamp for RPconveyance control. If the risk potential modification control B isbeing executed, then the controller 200 illuminates the indicator lampfor following distance control and flashes the indicator lamp for RPconveyance control slowly. If the risk potential modification control isin the cancellation phase, then the controller 200 flashes the indicatorlamp for following distance control slowly and illuminates the indicatorlamp for RP conveyance control. It is also possible to issue soundinformation and/or vibrate the accelerator pedal 71. After the commandvalues are sent, the current cycle of the control loop ends.

In the first to third embodiments, the risk potential RP is calculatedas the repulsive forces of two imaginary elastic bodies correlated tothe time to headway THW and the time to collision TTC of the vehiclewith respect to a preceding obstacle. However, the invention is notlimited to this method of calculating the risk potential RP. It is alsopossible to calculate the risk potential RP as the repulsive force ofonly one imaginary elastic body correlated to either the time to headwayTHW or the time to collision TTC. Still other feasible ideas includeadding a function of the inverse of the time to headway THW to afunction of the inverse of the time to collision TTC and using theresult as the risk potential RP or selecting the larger of the twofunctions as the risk potential RP.

The relationship between the risk potential RP and the reaction forcecommand value FA is not limited to that shown in FIG. 11. It is possibleto contrive the relationship such that the reaction force command valueFA increases as the risk potential RP increases. In the first to eighthembodiments, the accelerator pedal actuation reaction force control isexecuted based on the risk potential RP. The accelerator pedal 71 is thedriving operation device operated by the driver in order to drive thevehicle and the risk potential RP can be conveyed to the driver in acontinuous manner through the actuation reaction force. It is alsoacceptable to use the brake pedal or the steering wheel as thedriver-operated driving operation device and control the actuationreaction force exerted by the brake pedal or steering wheel based on therisk potential RP.

In the first to third embodiments, the RP conveyance control comprisesan actuation reaction force control contrived to control the actuationreaction force of a driver-operated driving operation device based onthe risk potential RP and a braking/driving force control contrived toimpose a target deceleration rate. However, the invention is not limitedto such a configuration and it is also possible to contrive the RPconveyance control to comprise only a braking/driving force controlexecuted based on the risk potential RP. It is also possible for the RPconveyance control to involve only a driving force control and not abraking force control.

In the first to eight embodiments described heretofore, the laser radar10, the vehicle speed sensor 20, and the frontward camera 30 canfunction as an obstacle detecting section. The risk potentialcalculating unit 151 and the controller 200 or 210 can function as arisk potential calculating section. The first target deceleration ratecalculating unit 153, the accelerator pedal reaction force controldevice 70, the engine controller 50, the brake actuator 60, and thecontroller 200 or 210 can function as a first driving assistance controlsystem. The second target deceleration rate calculating unit 154, theengine controller 50, the brake actuator 60, and the controller 200 or210 can function as the second driving assistance control system. Theaccelerator pedal stroke sensor 90, the steering switch unit 100, andthe controller 150, 200, or 210 can function as the transition detectingsection. The modified target deceleration rate calculating unit 155 andthe controller 200 or 210 can function as the control adjusting section.The accelerator pedal stroke sensor 90 can function as the acceleratorpedal actuation amount detecting section and the transition preparationdetecting section. The accelerator pedal stroke sensor 90 and thecontroller 150, 200, or 210 can function as the accelerator pedaldepression rate detecting section. However, the invention is not limitedto using these specific devices. For example, a milliwave radar of adifferent format can be used as the obstacle detecting section. It isalso possible to use only the engine controller 50 and the brakeactuator 60 as the braking/driving force control means or to use anentirely different means of decelerating the vehicle. In short, theexplanations presented heretofore are merely examples. When interpretingthe present invention, the invention should not be limited or restrainedin any way by the corresponding relationships between the embodimentsand the claims.

Thus, while only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. The functions of one element can be performed bytwo, and vice versa. The structures and functions of one embodiment canbe adopted in another embodiment. It is not necessary for all advantagesto be present in a particular embodiment at the same time. Thus, theforegoing descriptions of the embodiments according to the presentinvention are provided for illustration only, and not for the purpose oflimiting the invention as defined by the appended claims and theirequivalents.

What is claimed is:
 1. A vehicle driving assist system comprising: apreceding object detecting section configured to detect a precedingobject existing in front of a host vehicle; a risk potential calculatingsection configured to calculate a risk potential indicative of a degreeof convergence between the host vehicle and the preceding object basedon a detection result of the preceding object detecting section; a firstdriving assistance control system configured to control at least one ofan actuation reaction force exerted by a driver-operated drivingoperation device and a braking/driving force exerted against the hostvehicle based on the risk potential calculated by the risk potentialcalculating section; a second driving assistance control systemconfigured to control the braking/driving force of the host vehicle suchthat a headway distance is maintained between the host vehicle and thepreceding object; a transition detecting section configured to detect atransition of operating states between the first and second drivingassistance control systems; and a control adjusting section configuredto adjust at least one of the braking/driving force and the actuationreaction force during transition between the first and second drivingassistance control systems, the control adjusting section beingconfigured to give priority to the control executed by the seconddriving assistance control system over the control executed by the firstdriving assistance control system while both the first and seconddriving assistance control systems are in an operable state, the controladjusting section being further configured to execute the control of thefirst driving assistance control system while both the first and seconddriving assistance control systems are in the operable state and whilean accelerator pedal is depressed by a driver.
 2. The vehicle drivingassist system as recited in claim 1, wherein the operating states of thefirst and second driving assistance control systems are changed based onoperation of at least one driver operable switching device.
 3. Thevehicle driving assist system as recited in claim 1, further comprisinga transition preparation detecting section configured to detect apreparation state occurring before changing from a state in which thesecond driving assistance control system was operating to a state inwhich the first driving assistance control system is operating, thecontrol adjusting section being configured to limit the control executedby the first driving assistance control system when the transitionpreparation detecting section detects the preparation state.